I am GS on UA and Summit on F9 (Denver based). Both have their merits.
If you are 2P you won't get many upgrades and then I would pick F9 based on TV on board, free drinks (if you can make Summit) and pretty decent service with free cookies :-).

Also Summit gets free same day change and you have no ticket change fees (only difference in fare) so you get the best of both worlds (UA and WN) and it did help me out a couple times already when I had to do changes to flights. Also you can buy early and worst case just change them. Fares are usually also lower.

The only area where UA has a clear advantage is Mileage Plus. The miles on F9 are not that useful.

In your situation if you will not care about international flights and also will not care about getting the once in a while first class upgrade on UA then F9 will be the winner.