I'd say you've made a good decision. Maths is a well respected A Level and will go down well for most University courses later on.
At AS year, the whole year had to do Stats 1 but at A2 I had the option to do S2 or M1. I had opted for M1 but somehow I was put in the S2 set. Anyway, I tried it out for a week and found it to be simple enough so I just stuck with it. It's a lot more interesting than S1, which really was just GCSE to be honest. I suppose it'll depend on what you consider doing later on. Mechanics is great for a bit of a taster of what you might be doing in later years in Engineering based courses while Stats would be a taster for the Economics/Finance side of things perhaps.
I found the first year to be fairly simple. Core 1 was very much like GCSE but with Differentiation and Core 2 was a bit more hands on. Things really stepped up in Core 3 and 4 and at least for me, it required a lot of time to master. Stats 2 is a breeze. I finished my A Levels this year and came out with an A in Maths. I have no doubt that Maths at A Level was much harder in the past though.