I'm a FF member of AA and United. They're both ok, I prefer United when flying overseas, their seats are better on long haul aircrafts. AA is more professional, treat you as a valued customer, while you get more of a "one of the crowd" treatment at United. Since I spend a lot of time flying in and out of Dallas, AA is the natural carrier of choice.
Northwest have an old fleet and is struggling, so service there has been steadily declining since 9/11. Up until just a few years ago, they still flew old DC10 aircrafts accross the Atlantic! With everyone onboard watching the same movie. Worthless, I do however like the supernice terminal they have up in Minneapolis.
And Delta, oh, don't even get me started.

Infamous for cancelling many flights and total lack of service. At least do yourself one favor and stay away.
I think US Airways have their hub in Philly, meaning that they have more connections and so. Plus more check in counters and staff overall since it's their "home".