Get Northwest. Once you get status with them, you will get upgraded fairly regularly. You also get this benefit on Continental. Also, you can redeem Alaska and Delta points on Northwest. Between those three, you will always get where you're going. NWA also lets you get a membership for their lounge and you can use it on CO/DL/AS lounges. Thus, your membership is good at almost every membership. CO/CL/AS lounges give you free alcohol as well. The other airlines are a joke. UA and AA are willing to take your $400 for basically giving you nothing.

NWA also has plenty of mileage run available and you get access to plenty of carriers to find international travel. The only real drawback is no entertainment on their domestic flights, but if you travel a lot this should not be an issue.