I think it's pretty standard for most to do the MMath (MSci?) and usually you only supplicate for the bachelors later if you have a specific reason not to continue to the masters year (or you don't get the required results to continue to the masters year).
If you're a home student it's generally less complex to start on the longer course and swap to the shorter one later, from a SFE perspective.
Conversely I understand for international students, it's easier to manage the visa length requirements by starting on the shorter course then switching to the longer one.
Assuming you're a home student I would suggest applying to the MMath/MSci - it won't affect your chances of an offer whichever one you put at Oxford.