UA will give you more options out west. I was a longtime AA flier who switched to UA in the past couple of years due to western U.S. travel.

In terms of purely subjective softer touches - the customer service and unpublished courtesies - I felt that AA appreciated its Plats more than UA its 1P's. I was living in Chicago when I was an AA Plat in the 90s and I always liked the fact that United was the slightly bigger dog in town: I felt like it made AA appreciate its frequent customers a bit more. It could have been my imagination, or it could have been the awesome city ticket office near my office building that is no longer there.

Bringing CO into the mix is a big plus for UA - from ORD, it'll serve to give you some more options into NYC.

I'm currently a UA 1K and I'm at 100% on domestic upgrades this year. I have to use e-500's for them, but I earn more of those than I can use. I did okay as a mid-tier, but the best flights for upgrades would sometimes leave me back in the exit row in coach. (Still not a bad seat.)

ORD check-in and security always seems like a total zoo in the UA area. AA feels less chaotic to me.

Star Alliance allows stopovers on awards that cross regions. AA's new approach with everything being a one-way sort of kills off that feature. It's a trade off: FT'ers seem split as to which they prefer. I love international stopovers.

Of the global alliances, I personally feel that Star is the strongest and covers my destinations the best with the most options. Look at your desired destinations and see if that's true for you too.

Lots of pros and cons to both...