Pros and cons-
AA-
Platinum Service Centers are good and upgrades have less competition.
E+ throughout coach, not just 4 rows.
UA-
So many 1Ks in DC area, I don't stand a chance at my lowly Premier level. Still have yet to sit in E+ as company is pushing us to fly UA more.
More importantly, depends on where you fly to. Internationally, more flights on UA out of IAD and Star Alliance is more expansive in Asia. Domestically, AA has a mini-hub out of IAD to SJC, LAX and I believe SFO.
In your situation, I would qualify on UA and also take the challenge after July. You receive UA benefits through 2/03 and after July, receive AA benefits through 2/03 also. Being elite member on the 2 biggest airlines in the US, wow! I guess you can possibly have your cake and eat it too.
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"Fly me to the moon and let me earn alot of miles."