I'm not sure you have any choice in BOS for a lounge. You will (I assume) be flying AA, which goes out of B. BA is in E. The terminals are not connected, so I'd think you are stuck with the AA lounge. It is nice enough, but nothing special. There are some new restaurants in that terminal, so you could try a leisurely meal.

Honestly, though, and assuming that you are taking the 20:20 flight out of BOS, I'd hop in a cab and head into the city.