I travel with a carryon every single time. I got on something like 85 planes last year and I didn't check a single bag. Get a really good suitcase here - plan on spending US$200-300. I would recommend the Briggs & Riley 22" expandable suitcase myself.

Being based out of DEN United and the RCC will probably be what your choices are by and large. There are threads galore debating the RCC pluses and minuses on the United forum so help yourself to those. If you do decide to join, my suggestion is to wait until you build up some status on United - the price goes down quite a bit for membership (like $350 if you're premier exec).

In general if you're going to little podunk UX stations all the time, you're going to use the membership in DEN only so delays and however much time you want to spend sitting in DEN waiting for your flight. If you're flying to major cities, you'll be able to use it on either end and make use of it when things go south and you just want to get home (the major benefit IMHO). If you're traveling internationally and your employer flies you in business (C) and you wait a bit to make star gold (premier exec) status, you can generally not need a membership to use lounges on either end.