I don't mind Frontier's new seats for trips of 2-3 hours or less. They're a bit stiff and the style leaves something to be desired, but due to their design the pitch doesn't feel like it's that much worse than 31" on a legacy airline.

I've also never had a bad customer service experience on Frontier, the flight attendants in particular have always been wonderful.

I haven't flown Spirit much (and admittedly not at all in the past 2 years) but I do remember it felt much more claustrophobic than Frontier and the customer service in general was worse.