For me, it's primarily about if they fly where I want to go. A few years ago, most of my flying took me to the west. Frontier got the lion's share (had Ascent or Summit for two or so years, maybe a little more) with United picking up the slack. With Frontier's change in business strategy and my flying shifting to the east, United took the torch (currently silver). Due to a handful of other factors, Delta is currently doing its best to take the lead. Try to look at the big picture if you can and see what'll work best for you.