Since you already own a M-rod, go with the B-51.
The two are similar in weight and bulk, but the ergonomics of the B-51 are much better, mostly due to the lower sight line allowed by the no mag/no sleeve barrel.
Neither makes a very good woods gun, IMO... both are too long and heavy to be much fun in brush or carrying for long.
I found that the factory brake on the B-51 would accept several common size trim washers, alternating with 0-rings. Not near as quiet as the M-rod, but at 18 ft/lbs, not too loud.