The cheapest one.
We always try to get the cheapest flight rather than the refundable cuz we travel SWA often enough that if we had to change our plans we would just take the credit for future travel.
I found this info on the SWA website.
Refundable Any Time
These fares offer maximum flexibility and allow for changes to travel plans without a penalty.
* Fares do not include a federal excise tax of up to $3.40 that will be imposed for each flight segment of your itinerary. A flight segment is defined as a takeoff and a landing.
* Fares do not include airport-assessed Passenger Facility Charges (PFCs) of up to $18 and government-imposed September 11th Security Fee of up to $10 roundtrip.
* These fares are available for one-way or roundtrip travel.
* Reservations must be made at least one hour prior to departure.
* These fares are refundable and changeable.
* Ticketless Travel may be purchased online using any combination of 1 credit card, up to 4 southwestgiftcards, up to 8 Southwest LUV Vouchers, and funds from up to 4 previously unused or cancelled Ticketless Travel reservation(s).
* Southwest Airlines' travel vouchers, Tokens of LUV, paper gift certificates, and discount coupons may not be used as forms of payment for Ticketless Travel.
Special Fares
* Fares do not include a federal excise tax of up to $3.40 that will be imposed for each flight segment of your itinerary. A flight segment is defined as a takeoff and a landing.
* Fares do not include airport-assessed Passenger Facility Charges (PFCs) of up to $18 and government-imposed September 11th Security Fee of up to $10 roundtrip.
* Seats are limited and won't be available on some flights that operate during very busy travel times and holiday periods.
* Some fares may require travel on nonstop flights only.
* Some fares may require roundtrip travel.
* Some fares may only be combinable with other combinable nonrefundable fares.
* Some fares may require a stayover of at least one night.
* Reservations must be made from at least one hour prior to departure to at least seven days before departure, depending upon the city pair.
* Some Special Fares may be nonrefundable. Nonrefundable fares may be used as credit toward the purchase of another ticket.
* Ticketless Travel may be purchased online using any combination of 1 credit card, up to 4 southwestgiftcards, up to 8 Southwest LUV Vouchers, and funds from up to 4 previously unused or cancelled Ticketless Travel reservation(s).
* Southwest Airlines' travel vouchers, Tokens of LUV, paper gift certificates, and discount coupons may not be used as forms of payment for Ticketless Travel.
* Standby travel may require an upgrade to the full unrestricted fare.
Restricted Fares
* Fares do not include a federal excise tax of up to $3.40 that will be imposed for each flight segment of your itinerary. A flight segment is defined as a takeoff and a landing.
* Fares do not include airport-assessed Passenger Facility Charges (PFCs) of up to $18 and government-imposed September 11th Security Fee of up to $10 roundtrip.
* Seats are limited and won't be available on some flights that operate during very busy travel times and holiday periods.
* Some fares may require travel on nonstop flights only.
* Some fares may require roundtrip travel.
* Some fares may only be combinable with other combinable nonrefundable fares.
* Some fares may require a stayover of at least one night.
* Reservations must be made from at least three days before departure to at least fourteen days before departure, depending upon the city pair.
* These fares are nonrefundable, but may be used as credit toward the purchase of another ticket.
* Ticketless Travel may be purchased online using any combination of 1 credit card, up to 4 southwestgiftcards, up to 8 Southwest LUV Vouchers, and funds from up to 4 previously unused or cancelled Ticketless Travel reservation(s).
* Southwest Airlines' travel vouchers, Tokens of LUV, paper gift certificates, and discount coupons may not be used as forms of payment for Ticketless Travel.
* To travel standby on another flight, you must upgrade this ticket to the full unrestricted fare.
Advance Purchase Fares
* Fares do not include a federal excise tax of up to $3.40 that will be imposed for each flight segment of your itinerary. A flight segment is defined as a takeoff and a landing.
* Fares do not include airport-assessed Passenger Facility Charges (PFCs) of up to $18 and government-imposed September 11th Security Fee of up to $10 roundtrip.
* Seats are limited and won't be available on some flights that operate during very busy travel times and holiday periods.
* Some fares may require travel on nonstop flights only.
* Some fares may require roundtrip travel.
* Some fares may only be combinable with other combinable nonrefundable fares.
* Some fares may require a stayover of at least one night.
* Reservations must be made from at least three days before departure to at least fourteen days before departure, depending upon the city pair.
* These fares are nonrefundable, but may be used as credit toward the purchase of another ticket.
* Ticketless Travel may be purchased online using any combination of 1 credit card, up to 4 southwestgiftcards, up to 8 Southwest LUV Vouchers, and funds from up to 4 previously unused or cancelled Ticketless Travel reservation(s).
* Southwest Airlines' travel vouchers, Tokens of LUV, paper gift certificates, and discount coupons may not be used as forms of payment for Ticketless Travel.
* To travel standby on another flight, you must upgrade this ticket to the full unrestricted fare.
Senior Fare
* Fares do not include a federal excise tax of up to $3.40 that will be imposed for each flight segment of your itinerary. A flight segment is defined as a takeoff and a landing.
* Fares do not include airport-assessed passenger facility charges (PFC) of up to $9 and government-imposed September 11th Security Fee of up to $5 one-way.
* Seats are limited and won't be available on some flights that operate during very busy travel times and holiday periods.
* These fares are available for one-way or roundtrip travel.
* Reservations must be made at least one hour prior to departure.
* Fare is not combinable with Internet Special Fares.
* Passengers must be at least 65 years of age, with valid I.D.
* These fares are refundable and changeable.
* Ticketless Travel may be purchased online using any combination of 1 credit card, up to 4 southwestgiftcards, up to 8 Southwest LUV Vouchers, and funds from up to 4 previously unused or cancelled Ticketless Travel reservation(s).
* Southwest Airlines' travel vouchers, Tokens of LUV, paper gift certificates, and discount coupons may not be used as forms of payment for Ticketless Travel.
* To travel standby on another flight, you must upgrade this ticket to the full unrestricted fare.
Discount Fares
* Fares do not include a federal excise tax of up to $3.40 that will be imposed for each flight segment of your itinerary. A flight segment is defined as a takeoff and a landing.
* Fares do not include airport-assessed Passenger Facility Charges (PFCs) of up to $18 and government-imposed September 11th Security Fee of up to $10 roundtrip.
* Seats are limited and won't be available on some flights that operate during very busy travel times and holiday periods.
* Some fares may require travel on nonstop flights only.
* Some fares may only be offered on specific days of the week.
* Some fares may require roundtrip travel.
* Some fares may only be combinable with other combinable nonrefundable fares.
* Some fares may require a stayover of at least one night.
* Reservations must be made from at least one hour prior to departure to 21 days before departure, depending upon the city pair.
* These fares are nonrefundable, but may be used as credit toward the purchase of another ticket.
* Ticketless Travel may be purchased online using any combination of 1 credit card, up to 4 southwestgiftcards, up to 8 Southwest LUV Vouchers, and funds from up to 4 previously unused or cancelled Ticketless Travel reservation(s).
* Southwest Airlines' travel vouchers, Tokens of LUV, paper gift certificates, and discount coupons may not be used as forms of payment for Ticketless Travel.
* To travel standby on another flight, you must upgrade this ticket to the full unrestricted fare.
DING! Fare
* Fares do not include a federal excise tax of up to $3.40 that will be imposed for each flight segment of your itinerary. A flight segment is defined as a takeoff and a landing.
* Fares do not include airport-assessed Passenger Facility Charges (PFCs) of up to $18 and government-imposed September 11th Security Fee of up to $10 roundtrip.
* Fare is available only through DING! on southwest.com.
* These fares are available for one-way or roundtrip travel.
* Seats are limited and won't be available on some flights that operate during very busy travel times and holiday periods.
* Fare may be combined with other combinable fares.
* Fare is not combinable with Senior Fares.
* These fares are nonrefundable, but may be used as credit toward the purchase of another ticket.
* Ticketless Travel may be purchased online using any combination of 1 credit card, up to 4 southwestgiftcards, up to 8 Southwest LUV Vouchers, and funds from up to 4 previously unused or cancelled Ticketless Travel reservation(s).
* Southwest Airlines' travel vouchers, Tokens of LUV, paper gift certificates, and discount coupons may not be used as forms of payment for Ticketless Travel.
* To travel standby on another flight, you must upgrade this ticket to the full unrestricted fare.
Promotional Fares
* Fares do not include a federal excise tax of up to $3.40 that will be imposed for each flight segment of your itinerary. A flight segment is defined as a takeoff and a landing.
* Fares do not include airport-assessed Passenger Facility Charges (PFCs) of up to $18 and government-imposed September 11th Security Fee of up to $10 roundtrip.
* Seats are limited and won't be available on some flights that operate during very busy travel times and holiday periods.
* Some fares may require travel on nonstop flights only.
* Some fares may require roundtrip travel.
* Some fares may only be combinable with other combinable nonrefundable fares.
* Some fares may require a stayover of at least one night.
* Reservations must be made from at least one hour prior to departure to 21 days before departure, depending upon the city pair.
* Some Promotional Fares may be nonrefundable. Nonrefundable fares may be used as credit toward the purchase of another ticket.
* Ticketless Travel may be purchased online using any combination of 1 credit card, up to 4 southwestgiftcards, up to 8 Southwest LUV Vouchers, and funds from up to 4 previously unused or cancelled Ticketless Travel reservation(s).
* Southwest Airlines' travel vouchers, Tokens of LUV, paper gift certificates, and discount coupons may not be used as forms of payment for Ticketless Travel.
* To travel standby on another flight, you must upgrade this ticket to the full unrestricted fare.
Internet Special Fare
* Fares do not include a federal excise tax of up to $3.40 that will be imposed for each flight segment of your itinerary. A flight segment is defined as a takeoff and a landing.
* Fares do not include airport-assessed Passenger Facility Charges (PFCs) of up to $18 and government-imposed September 11th Security Fee of up to $10 roundtrip.
* Fare is only available for travel purchased through the Southwest Airlines web site.
* Seats are limited and won't be available on some flights that operate during very busy travel times and holiday periods.
* Fare may be combined with other combinable fares.
* Fare is not combinable with Senior Fares.
* Some fares may require travel on nonstop flights only.
* Fare is nonrefundable. Nonrefundable tickets may be used as credit toward the purchase of other tickets.
* Ticketless Travel may be purchased online using any combination of 1 credit card, up to 4 southwestgiftcards, up to 8 Southwest LUV Vouchers, and funds from up to 4 previously unused or cancelled Ticketless Travel reservation(s).
* Southwest Airlines' travel vouchers, Tokens of LUV, paper gift certificates, and discount coupons may not be used as forms of payment for Ticketless Travel.
* To travel standby on another flight, you must upgrade this ticket to the full unrestricted fare.
Even though all fares say you must upgrade your ticket to travel on another flight we have never been asked to do that and we have missed a few flights.
I hope this helps.