FWIW: My family is flying from K.C. to Orlando in February and we chose Midwest Airlines this time (have flown Southwest and Delta to Orlando in the past; have flown Midwest only once before, on a business trip to Toronto, way back in 1996 or 1997). Midwest was cheaper than Southwest at the time we booked the flight (of course, two days later Southwest cut their prices to a bit less than Midwest Airlines but I still would have paid the extra ten bucks for Midwest).
One possible drawback (depending on how you look at it) is that Midwest only had a late afternoon flight arriving in the late evening so we had to extend our Disney stay an additional day on the front end. If you want to drive to K.C. and fly out in the same day, an afternoon departure might work perfect for you.
One possible "perk" (depending on what seats you get) that Valeriekc didn't mention is that you will get reserved seats on Midwest as opposed to Southwest's open seating.
I did NOT do the internet searches that Valeriekc did but: When I was searching for February flights six weeks ago, one of the best fares I found was Delta but it was a mid-size plane (held somewhere from 45 to 60 passengers in five or six across seating, I believe, and it was a jet plane, not a prop plane). So, double-check that if plane size is important to you.
Also, if your travel dates are at all flexible, you might check prices for flying a day and two days before, and a day and two days after, your planned dates. My family's February dates could not be flexed anymore because of all of our DS's in school but if we could have flown to Orlando and back to K.C. one day earlier, our price per person would have dropped about $50 and, if we could have moved the trip up two days, our price per person would have dropped about $75.
-- Rob