I have flown Spirit, Frontier, United, American, Sun Country, and SWA in the past two years both locally and internationally.
I couldn't tell the difference other than I got a cup of water, soda, or a tiny bag of $1 over salted pretzels on UA, AA, and SWA. I normally carry my water bottle so I didn't miss anything.
Given that the cost of spirit and frontier can be significantly less than UA and SWA even when the cost of a checked in bag is factored into the equation (1 check in bag for 4 seats), and their planes are generally newer (Spirit), I have been known to fly them if they have non-stop flights from any of the two airports in Houston.