If they were both the same size, SRR was better and would win. Given that SRR came close to being a 175 lb champion. and Manny started at 112, this is a one-sided beatdown, probably inside the distance.
Sugar Ray Robinson is the first true speed and power combination puncher like Manny Pacquiao. But he's almost 6 ft and punch for punch far more stronger than Manny. This is one climb too many for Pacquiao. Even Manny knows his limits.