I am GS on UA and Summit on F9. F9 is a good LCC carrier for my needs (Denver originating) and the fares are very competitive. A lot of destinations and travel times are cheaper on F9 and I try not to set foot on a commuter jet with UA except if I have no other choice.

What status would you be able to reach since this can make a difference in your decision. If you can reach Summit (25k miles) then the perks are pretty good. Free change of even the lowest priced tickets (only WN offers that to my knowledge), free same day change (need to be 1K on UA) and the stretch seating is not bad. The free change is a huge bonus for me with frequently changing travel plans. I did get burned on domestic UA a couple of times.

You will not see first class but for most short haul flights thats perfectly acceptable. I do like the (free for Ascent or Summit) Direct TV and a free beer after a long work day is also not a bad deal.
In general if I am on my own dime I would pick F9 over your other choices except if you can at least reach Gold status with DL or US.
Award availability is ok and if you have status you can work it to your advantage. I got 3 one way's to Mexico during Thanksgiving week for 3 x 30k miles and then bought the cheaper other way.
A mile in the F9 is for me worth less then 1/2 of a United mile but I do use those mainly for premium class international travel. You will miss out on those and especially US has very good international availability if you want to travel business or first international. Also they run very good special deals when they sell miles with 100% bonus on top of them.