I assume that the difference is efficiency since the max t/o weights and fuel capacities are the same regardless of engine (except for the 757-200PF - freighter version???). The MTOW did increase over time after the introduction of the 757, but US has the highest weight versions (except the aforementioned PF's). While range at MTOW isn't that different (~200 nm) the range at given gross weights below max are greater with the P&W's. For example, for a range of 4,000 nm the RB211-535C's are limited to t/o weight of about 135K pounds, the RB211-535EB4's up to a t/o weight of 153K pounds, and the P&W's up to a t/o weight of 165K pounds. Those are for the basic airplane - adding winglets would increase all those figures somewhat.
Jim