We're AA flyers and pretty happy with it. The B terminal is rotten, but security usually isn't too bad and it looks like they are adding some restaurants and shops (nothing great) airside. A lot of MD-80's out of Boston, so not much in the way of better planes. Some better options flying to over the Atlantic, though.

Delta's terminal is fantastic and all the interaction I've had with DL employees in Boston has been great.

It's a close call, but I think I'd stick with AA just due to the better partners. It really depends on where you fly, though.

Mike