Welcome CaptainB:

If you're going to be flying a lot, you'll get the really major benefits by concentrating on one FF program.

Your profile says your from New York, so you'll have choices in Airlines.

I'd suggest getting a copy of the airline
schedules for AA and UA. And check it against the cities that you'll be going to.

Also, webflyer has summeries for the various programs. Estimate how many miles and/or segments you'll be flying and compare the elite levels for each program.

Another thing thats major for me, is how many direct flights does the airline offer for my trips. I fly a good number of NYC-STL trips a year which helped me choose
Continental as my primary airline.

Another big issue is partner's. Look at each Airlines partner list. AA has an alliance with US air. UA is alligned with Delta.

Catman's suggestion of going thru the posts in the different forums is a great one. I did that when I first came to webflyer.