I know it's hard, but you should try to figure out approximately how many miles you will earn so you can figure out what elite level you'll qualify at. I think you need this information to compare the elite benefits of the various airlines - upgrades, club access, bonus miles.

Nonstop analysis:
BOS-MCO: DL, AA, B6
BOS-ORD: AA, UA
BOS-SAN: AA, AS
BOS-SFO: UA, AA, AS
BOS-DEN: UA, B6
BOS-SEA: AS

According to this thread, Delta will be moving to a great looking new terminal at BOS.

How important is airport club access to you? Comparing UA and AA: It looks like both AA and UA have clubs in BOS. Certainly both have clubs in ORD. UA has a club in MCO.