Well aside from the extra year in MMaths I'm wondering are there any huge advantages of MMaths over a BSc? I've decided I want to do Maths after College already and if it makes any difference I prefer Pure mathematics over all other types, I'd like to apply to do a MMaths degree but it's an extra year out of potential work and I'm assuming a BSc would still be reputable enough to get you into a nice job based on Mathematics,
Apparently, the first two years are very similar. In the third year, if you are good enough, you can go into MMath and study more advanced stuff, if not, you just do the Bsc Maths. Therefore, the MMath is 4 years, the Bsc is 3. If you love maths and are very good at it, go for the 4 year course.
The point of the whole MMath thing is that it is a funding convenient arrangement for people who would have otherwise have done a three year degree and then a Masters. The fourth year is normally similar to a taught Masters (albeit with a less substantial dissertation element). The first three years will invariably be the same on either course (although you might do some of the fourth year stuff in the third year and vica versa)
I wouldn't worry about it now though as normally, you should be able to sign up for the three or four year course but still have the flexibility to decide which one you want to do when you get to your second year or so.