From all of my readings, I don't see substantial differentiation between all the the majors. It seems prudent to stick with the one that one is familiar with, as long as it works. I know AA, and that works for me, so I stick. I like the EXP desk and the SWUs, but other majors have different features that are better (UA E+ is nice, for example).
Cheers.
Just looking at the threads on FT for the major legacy domestic carriers I'd say overall its a wash but one carrier may be more to one's needs. Also, are factors such as routing and number of flight options from one's home airport. I just wonder how long FF programs, specifically elite benefits, are going to be based upon criteria from years back but that's another thread.