Over the past 4 years, I have flown frontier, spirit, American, United and Southwest. I didn't see any difference other than the cheap snacks and a can of soda the more expensive airlines offer.
In terms of cost, I used to be able to get spirit and frontier for significantly lower cost even with the purchase of one checked bag for the family. These days, spirit pricing has gone up although there are still significantly cheaper depending on destination.
One other thing, when spirit stopped offering direct flights to international destinations from my nearest airport, both United and Southwest (two with the most direct flights from my nearest airport) did not feel the pressure to reduce their price. Now Southwest is the most expensive followed by United. Spirit have newer planes than United, southwest, and AA (at least the ones I have been on).
Here are examples of return flights I got with spirit and frontier departing either of the two Houston airports:
BWI - $80
Denver- $70
Orlando - $50
Cancun - $150
San Jose (Costa Rica) - $168