Probably my answer as well. I think unless you live close to LGA the travel time is probably comparable on the Acela and way more comfortable. It's almost a no-brainer from NYC to Washington at 2:45 on a fast Acela. It's a bit longer to Boston so a bit less obvious to do the Acela. But the Acela is way more comfortable and has free WiFi on board. I seldom fly between NYC and either WAS or BOS any more.

If you insist on flying the Delta and AA shuttles are pretty comparable. One leaves on the hour and one on the half hour as I recall. Take whichever one works best with your schedule.