Customer service wise they both suck IMO.

I fly UA/CO and US often to earn US Dividend Miles. From a silver elite stand-point my only perk is free luggage, priority security lines and priority boarding on all three airlines. Plus the occasional UG on US flights and no same day move up change fee.

It really depends on what your ultimate goal is. Perks and/or miles.

My "goal" is to earn enough miles to take my family of 4 to Hawaii when my daughter graduates from H/S in two years. Being silver gets a 25% bonus and I'm really pushing it to get gold status by year-end for the 50% bonus miles on *A carriers.

Good luck in deciding and safe travels to you.