DEN is an extremely reliable airport, even in winter (my sig notwithstanding!); it's also by far the closest to GJT and has the most frequency to/from GJT. United runs a mix of ER-145s, CRJs and even Q400s on this route. I vote for the ER-145; you can also get that on AA ex-DFW.
I think one more interesting consideration is: Do you have a current preferred airline upon which you've built status?