I you live in, and will never in your life, move out of LAS, DEN, MCO, I would go Frontier.

both are single type airlines
both are LCC or ULCC business model
both you can make a crap load of money as a Captain
neither are perfect
both have brand new or near-new airplanes coming
1 is Airbus, 1 is 737
1 offers 4 year or less upgrade, 1 does not

If you live in or near, and will never move out of, OAK, FLL/MIA, LAX, MDW, DAL, HOU, SAT/AUS, (Texas or Oklahoma in general), I would do SWA

my opinion