What's your expected annual travel total? What routes do you expect to be flying? Are you into minimizing time spent on the plane, or are connections that would allow you to maximize mileage/elite benefits OK? What do you want to do with your miles?

Giving you an answer without knowing those things is an exercise in projecting our own biases and experiences onto you. Both US and UA have their strong and weak points as programs. Knowing more about the kind of travel you expect to be doing will help inform you.