Having flown Frontier very recently on a similar length flight - PIT/ORD, (+ RyanAir & EasyJet) I'd say that, if you're taking a short trip of a day or 2, and can carry on your bag, and you're not worried about an IROPS situation, Spirit should be fine, particularly if you want to spend the extra $$$ in LAS!
If your situation involves checking bags, and you're a DL Medallion, flying DL might make more sense.