Much of what program you join depends on what airline you like. I like NW/CO so the NW program is a very good fit for me. As a Platinum, I get 125% bonus miles, free upgrades in North America, and a lot of good promotions.

You won't get free upgrades on AA, only 4 500 mile upgrade e-certs once you fly 10k miles. As a Platinum on AA, I found their phone agents unhelpful when mechanicals occurred. They do have good award availability, but their Gold level only gives you 25% bonus miles, not 50% of NW/CO/DL and others.

I'd research the various threads before you jump in. If you do go for the AA program, I'd go for the Platinum challenge (Or register for both Gold and Platinum if that's still possible). Make sure you read the current thread, as there have been changes about what flights qualify.