The 320 has been known to swap into a few JFK-BOS-JFK flights. Peak travel times you will see a bus and during the down times you might see a 190. It is going to depend on the requirements of the company.

If you want to be on and off quick you can't go wrong with sitting up front. The middle of the cabin has a tendency of getting warmer than the rest of the cabin because of the fuel flow and you are right in between cabin temperature zones. I always sit in the back only because i non-rev on that plane all the time ORD-BOS and it is usually less full back there, most non-rev flight attendants sit back there as well so we can socialize with the working crew. It can get crowed in the back as it is rather small and if a line forms for the bathroom you may have the line extend past you which could get annoying. I would say take a seat up in front 2-7 would be the best. There isn't any big difference in leg room, you will still sit comfortably!