Benefits for the two levels are nearly identical, with only a few differences:

AA: You're admitted to the Platinum Service Centers
UA: Sorry, only our 1Ks can have that level of customer service

AA: You get Platinum luggage tags
UA: Sorry, only our 1Ks deserve luggage tags

AA: Free access to Platinum Hotline from inflight telephones
UA: You're kidding, right?

However, from a FlyerTalk perspective:

AA has deeply offended us.
http://www.flyertalk.com/forum/Forum71/HTML/003070.html

UA supports our community.
http://www.flyertalk.com/transcripts/041201.html

Choose the lesser evil. Go with United.