That's a rather broad question! Are there any features that are of particular interest to you, such as domestic upgrades, elite bonus, etc...?

You might want to take a look at WebFlyer's review of each, available here:

AA: http://www.webflyer.com/@reviews/program.php?key=AA01

NW: http://www.webflyer.com/@reviews/program.php?key=NW01