From
The Boeing Company, MTOW of the:-
757-200 is 115,660 kilograms.
757-300 is 123,600 kilograms.
Which are both below the 136T threshold so are classified as a M. However, in the UK I believe the difference is that min spacing on approach is 4nm with a 757 ahead and a medium behind. No difference on departure.
As an aside, I have had worse wake from the old A320-100's (no winglets) than I have encountered from a 757!