take as much diff eq and PDE as you can stomach, then take some more. Along these lines, I would also suggest taking linear and non-linear dynamical systems. I was amazed how much insight I gained from a single course in non-linear dynamics. Really helped me visualize what is going on behind the scenes and actually cleared up a lot of things I really didn't get from linear.
stat is good to have, but honestly, I am doing fine on just intro-stat. Everything else stat related that I've needed has been covered in other classes.
Some sort of numerical methods course would also be beneficial. Even as a pure physics major, you will not be able to avoid computer simulation, and at least understanding how discrete solvers work and their limitations is important for anyone seeking a degree in science.
Honestly, unless you are contemplating a grad level course in game-theory (even that may have applications), I don't believe there is a math class you can take that won't help you in some way. both fields are looking at the world through math-colored glasses and the better you are able to imagine the world in this fashion, the better off you are.
honestly, I would compare the required classes list between the two and take the overlapping classes as long as possible until you make a decision. If you wind up finding a math course that you want to take, but the credits won't count towards your degree, just ask the prof if you can sit in on the lecture. You still get the exposure to the material but you don't have the added homework load (and you don't have to pay for it).