I agree with JGR01's post on most accounts, but I'd caution you about taking the comment about NW's "general lack of customer service" too seriously (if NW is one of your possible options). It's possible (JGR01 can correct me if I'm wrong) that the comment is from someone who is used to the elite services on AA, and then flew NW as a non-elite customer.

I can't comment on NW's service if you aren't elite, but if you fly enough to achieve Gold or Platinum status, I can tell you that their customer service is outstanding. There is a dedicated reservations/customer service line just for Gold and Platinum members, and I have had a great outcome every time I have called. Further, they have great electronic (phone, email) notification systems that you can sign up for, so you receive proactive notice of service issues, like delays or cancellations. No, they aren't perfect and I wish the delay notices came out earlier, but it's still far better than what I have experienced on other domestic airlines (admittedly as a non-elite).

If much of your flying is domestic, the other thing about being elite on NW is that you will probably get more free domestic upgrades than you would if flying on other airlines. Over the past year and a half (mostly as a Gold, just became Platinum), I have been upgraded on about 80% of my domestic flights -- despite the fact that the planes are fuller than ever.

Looks like neither JGR01 nor I can give you a current comparison of AA non-elite vs. NW non-elite. Just wanted to make sure you see the NW elite picture to compare with his description of the AA elite one....

Again, there are lots of factors, but the most important is where you'll be flying and which airlines have the best routes to meet those needs.