I'll second what Mr M said. It is a matter of opinion.
I have taken M1, S1, M2 and just taken M3 today. With mechanics it is purely a matter of good training in techniques of answering questions:
(if in doubt DRAW A DIAGRAM!) and although M3 may look harder (to some) it relies a lot more on thought and is less prone to calculator errors.
But seriously if you did well in M1 and M2, I wouldn't have said it is that big a step as some people have said. The same principles apply although it should be said that M3 does rely quite a bit on C3 and C4 work, so if pure maths is not your strong point, S3 is a much better option.
As a converted Mechanics mathematician I will have to say that M3 is a lot more interesting and diverting than manipulating numbers to make them say what you want! But then again its a personal choice (just choose M3!).