I agree with the above poster about trying both programs. I don't fly AA frequently, so I can't comment on how they are for the elite level traveler. For me, personally, the extra legroom in UA's Economy Plus (E+) is a big selling point because I'm 6'-4" and every extra inch does count!
As an elite flyer, you can select E+ seat assignments when booking. As a non-elite you can purchase a year of E+ seating assignments (I think it's $300), or purchase full fare Y (yeah, right). I really notice the difference when I have to fly on AA, DL, CO, HP, WN, etc. in coach.
I've been elite with a couple of other programs over the years and I can say (again, for me personally) UA's customer service is the best I've experienced. And I seem to get a lot of upgrades, but as they say, your experience may vary. Just my opinion of course. There have been ups and downs, but overall I'm been very pleased with UA.
So welcome to the forum. Let us know how things go, and safe flying to you.