well as been said above,M1 depends alot on physics
assuming tht u hv already don ur AS physics i recommend u to do M1 because it'z very similar to mechanics part in AS physics.frthermore M1 is very easy once da basic concepts r graspd well and if ur planing to engineering,specialy mechanical engineering, M1 helps alot and puts more points than doin S2.
But on the other hand,u hv already done S1,so S2 shld be very easy to u cause S2 depends alot on S1 which u hv don @ AS.actually in S2 has been officialy taken as Further maths @ my collge to give M1 as the maths unit fr non-Further maths students!
So it's all about wat ur good at,did u get good marks @ S1 than fr physics 1?? than do S2 else do M1