DL is bigger at BDL than everyone else, but UA is probably the way to go over AA. Instead of taking ER4s and CRJs (soon to be ER4s, AGAIN) to ORD you can take an A320 with a decent daily schedule. I don't care for the sparse sched AA mainline has to 3x DFW, 3x MIA and 1x SJU either from there.

I'd take a look at Bradley's wiki page destination list for UA and DL and factor in convenience. If award redemptions are important to you, UA over DL