I live in near MCI, a non-hub where I can choose from any of the majors.

UA: I do a lot of Chicago/Denver travel, leaving UA as the only choice with frequent nonstops to both cities. My experience with them hasn't been too bad - even the summer from hell didn't affect many of my flights because I fly mostly earlier in the day. It seems like EVERYONE is Elite, though.

US: I fly US for all other travel whenever possible. I got started on them flying MCI-DCA, and found that I rarely waited in long lines on either end (coach, non-elite) and found that I was rarely on hold when I called their customer service line. These two things alone are more important to me than almost anything else. I switched a lot of travel to US, achieved elite status, and found better-than-average upgrade opportunities - partially due to a smaller number of Gold Preferred travelers with which to compete for upgrades (compared to UA PE or AA Plat). Also, US tends to kick in courtesy upgrades from time to time - even on transatlantics. Final bonus: mid-tier elite earns two systemwides on US; zero on AA or UA.