I agree with others here: as a general rule, achieving the single highest tier status you can within a major alliance is superior to holding several low-tier statuses. The only exception would be if you're absolutely forced to split alliances because of destinations - which is rare these days and as you've said not the case for you.

Unless AA is consistently giving you better schedule options than US/UA, I'd probably stay where you are. I've been mid-tier on all three of those (holding US GP status long before US allied with UA) and I enjoyed the service and upgrade frequency I received on US. There are other things I prefer about AA (more mainline jets where I want to go on short domestic hops) and UA (E+ and better global service where I want to go), but if US is already serving you fairly well now, there's no compelling reason to bail.