There's an argument to be made that now is the perfect time to gain status and earn lifetime miles with UA since you have nothing to lose with AA (i.e. you'll still have VIP status with the airline). I'd try getting UA to
status match you which based on the AA level you have you'll at least get Premier Platinum off the bat which entitles you to play the upgrade lottery, lounge access (when travelling outside the 50 states), Economy+ seating and complimentary Marriott/SPG Gold (if you don't have that already) via the
RewardsPlus program. You'll also get many of these priority benefits when travelling on *A partner airlines like Air Canada, Lufthansa and ANA. Once you hit 1K, you get a free snack & beverage for every UA flight you take which when combined with exit row seating in Economy+ can very closely mimic a domestic F experience. To keep Platinum you then need to rack up 18,000 PQM which I suspect you'll pull off in a month
I fly a decent amount with UA and am consistently pleased with the onetime performance of the airline (have had a couple of hiccups out of SFO and ORD but that's part for the course). I find that when IRROPs do happen they treat their VIPs well - as a lowly UA Gold when a flight I was on went MX, UA put me up in a hotel gave me around $80 USD in meal vouchers, rebooked me on the J flight I want (I was upgraded to from Y using the CPU lottery) and covered all ground transportation costs. I doubt AA would treat their Gold or mid-tier Platinum members as good as that.
Do I have gripes with UA? You bet! For one thing their domestic lounges don't have showers, something that AA and DL seem to be adding to their lounges. Their food spread in the lounges also ins't that great albeit at least you don't have to pay for food as in AA. As others have pointed upgrades on UA are going the way of the dodo bird. As a lowly Gold I've only received 2 CPUs so far (ORD -> YYZ and BQN -> EWR) out of around 15 UA flights that qualified for upgrades. Also we've heard reports of people who hit Platinum/1K not being able to have their upgrades clear using certificates (i.e. SWUs, called GPUs in UA speak) due to full J cabins and/or the countless ToD offers non-elites get. One other benefit to UA is sometimes you get decent buy up offers (i.e. I recently received a $1024 USD offer to upgrade EWR to NRT from Y to J which would also include bonus mileage - I declined cause I'm a cheapskate).
To summarize : AA is likely the better option if you prefer upgrades over direct flights and on-time performance. UA is better if you want to attain status on another alliance and find the perks tied to Premier Plat/1K compelling enough to forgo certainty in upgrades.
Safe Travels,
James