I'm not the most experienced with this, but this season I was helping a friend/ coworker track and pack out a decent size black bear that he'd shot. He used a 30-30 for the first shot, but had hit pretty far back and the bear ran into the underbrush. When he called me, we both tooled up with the biggest repeating guns we had: for me, that was a 30-30, and for him that was a 45-70 with 350 grain hard casts, backed up by .44 magnums.
We found the bear before it found us, and one shot from that 45-70 put him belly up.
I'm not sure where I was going with that...
I's say use whatever single shot you are best with, and have a buddy with a 45-70 repeater.
--TK