I would have to agree with those who say to pick the one that most suits your travel needs. I am a member of both but have more miles with American. Being that I am in the travel industry, I cannot get miles with the discounted tickets I purchase from UA but I recently went to Chile on a free ticket and was credited over 12,000 miles. I have seen more opportunities to earn AA miles from Kelloggs, MCI Worldcom, and the free gift certificates from More.com (redeemable for AA travel vouchers).
There is no need to limit oneself so go for what you can and strive to get all that you can to travel well.