I've done both module really it depends what you like. If you found M1 really easy and simple I'd learn M2 and you don't learn many new formulae in M2, just new applications. If, however, you didn't like or do well at M1 I would choose S1, as it is really easy.
If you do choose M2 I'd ask for an S1 book nonetheless just so you can look through it.
Personally I would do S1, but I did further maths so we did both anyway.