First of all a four year MMath is different from a BSc+MSc. Two main differences (i) MMath is covered by the usual loans/grants scheme as a BSc whereas its hard to get any funding for an MSc., (i) MSc courses usually go till the end of the summer with a more substantial dissertation than the MMath. The MMath certainly wins over the BSc if you are good enough to do well at it (eg if you fall below a 2.1 you should usually bail out with a BSc). MMath graduates tend to do much better in the (standard graduate training scheme) job market than BSc graduates for a variety of reasons. To some extent MMath graduates with a well chosen final year project can compete with MSc graduates for technical jobs (ie start doing something proper not just training). Both can be a good start to a PhD but on the whole an MSc (with its longer dissertation) is a better preparation if it is in the right area. Another reason to do an MSc is a fresh start (eg you got a 2.1 but felt you were worth a first) or a change of direction (eg an MSc on something really specific). In statistics an MSc is still seen as the entry level qualification to the profession, although some MStat or MMath in statistics graduates might be able to blag their way in.