OP didn't say he was looking for the cheapest alternative, he said he was looking for least chance of delay and some comfort.

The Acela will be neither cheaper nor faster unless you hit the time just right. While that is through on the micro-hop to DCA, the BOS trip takes longer.

I would not fuss much between DL, AA, and B6. The flight is so short and the service so limited, that there is not much to be gained by one over the other. If everything else were exactly equal, I would choose AA, just because it has better backup and is more likely to swipe an aircraft from another service to service this hop.