With M1, each paper is almost the same, there's a limit to what kind of questions they can and will ask so if you do enough practice you can guarantee yourself an A. In my opinion half of the paper doesn't even require any thinking, because there is always going to be something that's been in a past paper. I think you'll understand more once you do it. Once you get it, its very easy to secure very high marks, I don't feel this is the case with S1.
C3 and C4 can be a bit more unpredictable, the content isn't actually that hard but the exams can be more demanding as new things tend to come up more than with M1 (and all of AS maths for that matter)