I'll let others take a shot at distinguishing features,as this is a good complex to learn. I'm just wondering when this photo was taken- was it taken in MA? Was it taken in salt,fresh water..mix? These two are important things to consider when considering Long-billed Dowitchers in the east.
The patterns on the tertials of the left (short-billed) don't exist on the right bird (long-billed) in this plumage...buffy breast on short-billed and much duller gray breast on long-billed. Slightly less well-defined white supercilium in long-billed.