In my opinion, both are very good schools, and you will receive a top notch education wherever you go. I think in the end, your choice should depend on details like which city you prefer to live in, university size, whether you're interested in UW's co-op, etc.
I'm headed for UW this September into the math faculty myself, and although I'm happy with my decision, in retrospect, I probably should have looked around more in terms of universities. I didn't even apply to UofT.
By the way, are you talking about choosing schools for September? I thought OUAC needed you to make this decision back in June for the upcomming semester.
Oh, and on a passing note, there's a bit of a difference between faculties in UW's mathematical physics program. If you're going through the math faculty, you'll focus more on math/CS classes alongside with your physics classes whereas the mathematical physics program through the science faculty will involve more labs and other science courses as opposed to math. Although, I was looking through the first year classes, and for the most part, they looked the same no matter which faculty you were in, so I'd imagine that it wouldn't be too hard to switch faculties after first year, assuming that you're sticking to the mathematical physics major.
cheers