Definitely pure math. I spent a ton of time in college studying linear algebra and abstract algebra, and I remember very little of it. The issue with the advanced classes is that the time investment is enormous (1 class took more time than 4 others combined), and they're not worth doing halfway. I think if I'd stuck to computer science, I would have graduated a stronger programmer, and if I'd taken more sociology/art classes, I would have had more fun.

A lot of people talk about beauty in math, but the beauty part of it never clicked for me the way it did for art. I think there's a sterility about proofs and numbers. If you don't love pure math, you will grow to hate it.