Yup, my hunch is that UA is # 1 in availability (nudging out AA). It's close enough that I can't be 100% sure. I can be 100% sure that a flyer used to redeeming points on UA would be unhappy if he/she suddenly had to find availability on DL.

I'm surprised this issue hasn't gotten more attention. Being able to actually find seats when you need them makes the awards significantly more valuable. I've had situations with the "lesser carriers" where they've offered $99 (or less!) transcons and not had seats on the same flights available for standard reward travel. That's idiotic.