I switched from UA to AA as my primary carrier three or four years ago. At the time my main reason for switching was the fact that the 100% elite mileage bonus in AAdvantage was good on all Oneworld airlines and all AA codeshares on any airline. In the case of UA, the elite bonus was valid only on UA codeshares with LH, and not on any other UA codeshares or Star Alliance partners. It also helped that AA had non-stop JFK/GIG service at the time. Although that’s no longer true, AA is now one-stop, while UA is two-stop (with a plane change too). There were other considerations, now no longer valid: the exchange of domestic upgrades for 2,500 miles (now only 500 miles), very reasonable Oneworld around-the-world fares (still better than Star Alliance). Even though AA is working diligently at chipping away elite benefits, it seems that UA is doing a better job of it. So, I’m staying with AA. At this point, there doesn’t seem to be very much choice.