be careful with UA lounge. You can use it ONLY if you are flying UA flight that day.
not worth the expense if you will be flying a hodgepodge of airlines.
It seems like most of your flights will be short (Denver to west coast cities)
Jetblue is really good and you can get their elite level and get some benefits out of it.
Southwest has a nice perk of letting you name a co-flyer that can come for almost free on any flight you take with Southwest once you have hit 100K miles. In the past (not sure if still does), it allowed you to count the bonus miles you got via credit card sign up.
The advantage of United is the fact that those miles can be used for lots of international travel (assuming the contract between your company and UA allows you to collect the miles) and their lounges are nothing special but always better than hanging out in the terminal. if you hit Gold with United you get to bring more bags if needed and you get some perks with Star Alliance.
I would also consider how much time you are on the road. Once the excitement wears off then you will be looking/wanting the quickest shortest and no connection flight to get back home and screw points and miles.