As a freshman at an IB School in IB HL 1 Mathematics currently, I can vouch that taking IB SL or HL is much more important than taking Math Studies. Especially if you plan to be majoring in any field that has the slightest dealings with math (all sciences, economics, etc.) I have a couple friends taking IB Math Studies and what I learn simply outclasses what they do by a factor of a thousand (they find it incredibly boring).