Although I prefer United - if I were in your position, I would get a status match from Northwest WorldPerks or their partner Continental OnePass. United more than likely will not match your status, however OnePass and CO will - and that status is good for both NW, CO & to a lesser degree America West.
CO/NW elites receive free upgrades without certs (like UA requires) when available. So I would continue to fly qualifying AA segments, whether on AA or a partner, and get your status match at CO. I did this last year and it has worked out great.
As a side note, just make a generic letter and insert -blank airline name- and send it to everyone and get them to match your status. You never know when you'll get stuck on another airline.
------------------
Ken in Sacramento