Taking the non-stop (especially something short like BOS-LHR) nearly always beats a connection involving JFK. Not a lot to choose between BA, DL and VS for the nonstop, so the lounges at either end might be the deciding factor (in descending order or preference, VS Clubhouse, BA Galleries and, a long way back, DL SkyDump).

If you really want to connect and stay on oneworld, I would favor a connection in PHL over JFK any day. Especially if you're originating in MHT (i.e., avoid Logan altogether).