Add me to the pro-AA consensus for your situation. Since your situation ain't broke, why fix it?

I'm AA EXP and UA 1K, and while I see the two programs and airlines as more evenly matched than some here, the bottom line is that you're doing great with AA upgrades and reliability. In addition, the UA program has deteriorated somewhat in recent years, and may continue to do so.

Plus you have AA EXP status already, which can come in very handy to get you home on time during irregular ops. Building up equivalent status on UA or Delta means sacrificing upgrades and not being well taken care of on those airlines in the interim until you achieve top status.