I did Edexcel M1 and MEI S2 so I can't comment specifically on MEI's M1, but for me at least M1 and S2 were both fairly straightforward modules, comparatively speaking. M1 is probably easier and it helps you get better in Physics, too. S2 is conceptually harder than S1 (again I did Edexcel S1 and MEI S2 so take my comparison with a grain of salt), but the S2 exam is very straightforward, there are never really any surprises on the exam and the questions are less annoying - it's mostly just probability distributuons (I think you've seen Binomial before? Whereas in Edexcel S1 we did the normal first) and hypothesis testing (which I think is also in MEI S1 and not Edexcel). No tree diagrams, venn diagrams, counters in bags, et cetera. Much less obnoxious, less thinking outside the box.
S2 was really good for me, but I did it as part of my further maths. I'm not really sure it compares in simplicity to M1, which is, at the end of the day, basically an AS maths module. Your choice really. You can probably afford to go with whatever sounds more interesting to you, but M1 is probably the rational choice if you want to help out your physics too.