Math is more versatile. Finance is more job specific. Both are respected and valued, so look for ways to incorporate both in your studies.
My path to an investment career was Math (and Econ) undergrad, then MS in Finance.
An investment career does not use all branches of math equally. I used quite a bit of Algebra and Statistics, occasional Calculus, and almost no Geometry or Trig.
Some universities have a quantitative Finance option. Perhaps you could tailor your own?
If you want to teach college level Math, a PhD is likely required.
Courses that might complement your interests:
- Accounting (both Financial and Managerial)
- Computer Science (Excel, database skills)
- Economics (both Micro and Macro)
- Business Law
- Business Communication