When I flew Frontier in October for a
Disneyland trip, by the time I added in the additional charges for seat reservation ($12 didn't opt for the better seat at $24), baggage fee (checked was cheaper than carry-on fee but still $30); the pricing was very close to Southwest's ticket price. I know Frontier increased fees again in December, so double-check current pricing. Be aware that you pay for every little thing on Frontier except for one personal item that must fit under the seat in front of you.
Also Frontier does not participate in the Airline Resort Check-in.
If you use the Magic Express for your return to MCO, you'll have to handle your own bags. I pack heavy for WDW trips. So SW's two bags fly free plus having the bags checked in at the resort and no need to deal with them until I'm at home is a big plus for me.