Rahman at his best would win by knockout. Try to imagine Bruno beating Sanders and Lewis by stoppage back to back.
The smart money would be on Rahman, though he knew how to f*** up on the day. At the end of teh day however, you have to look at both men's results. Bruno was generally coming ups short against the very best, while Rahman was winning some and losing some against them. Bruno was pretty consistent, while Rahman on a good day, is a very different creature to Rahman on a bad day.
I think Rahman is more resilient and clocks Bruno with those overhand rights, stops him on his feet around Round 9.
You have to back Rahman at some point to catch Bruno and turned him into a stunned rabbit in headlights, that would have no way back. Bruno was a classic on top fighter who was not able to ever beat a world level fighter other than McCall, who was by that point severely declined.