Here's an excellent thread on this topic written by an FT'r who is both 1K on UA and EXP on AA. It is a bit dated, but many of the changes are summarized in the last few posts of the thread.

I'll first give you my standard disclosure statement: I am a former AA employee, been both a union member and a manager of a union group, lastly as customer service manager of both passenger and ramp services at one of AA's largest non hub stations. Left the company of my own freewill and now I'm a card-carrying PLT on AA and a 1P on UA. But am mostly an unappologetic AA loyalist even when I think they're screwing up.

I fly both about 50/50 with a little of that 50% on UA actually flown on US but credited to my UA account. What I'm about to say is nothing but my opinion so take it for what you will. If you're going to be flying CX any at all, I would probably go with AA and OneWorld and work on keeping that CX status if that's a goal. While I personally prefer UA's J seats to AA's, my opinions may change with the advent of AA's new J seat whenever it makes an appearance. Domestically though, (and I'm guessing you are a west coast person), UA has far more service up and down the left coast. UA doesn't have MD80's, and they do have Economy Plus.