I am guessing that you're year 12 (or younger) as the current year 13 admissions round has just completed. If so, I think the best thing would be to visit each university on open days if that is possible - getting a feel for the cities and the two departments would be best. Other than that, for the straight maths degree, there is much more alike than there is different. The Cambridge course has a slightly more applied feel (marginally) and if you're interested in joint degrees (with philosophy, statistics, or computer science) then Oxford has options there. They both offer routes into theoretical physics, but in very different ways. The biggest difference is really in the application process. Happy to answer further questions.