Just to note, you posted in the US forum, so obviously the posters are skewed toward that airline. I really don't know enough, myself, to join the conversation. I've taken JFK-BOS on B6 many times, but you didn't ask about that option though you should know it does exist.

I've taken DL shuttle once on a Sunday afternoon. Just like Southwest, people line up early by the gate as there is not assigned seating. That's really the only big draw back, in my eyes, about the DL shuttle.

Should DL and US merge, the shuttle market will get interesting. I imagine AA and B6 would both my major contenders to get those shuttle spots.