Oxbridge is unrivalled for mathematics. I read that Oxford's has 14 fellows of the Royal Society (one-in-six of the mathematical fellows worldwide), now it has 18. Cambridge is similar, because I also read that Oxbridge accounts for 1/3rd of all such fellows. THES yet again put Oxbridge on top for science, and I would reckon that for maths as well. America's biggest pure-mathematicians are all formerly from Oxbridge (Wiles, Kronheimer, Taylor etc.). Oxford however, has the larger pure maths faculty of the two, as opposed to the applied for Cambridge. Don't move out of Oxbridge circles, unless for experience's sake, or if you can't take it. Afterall, Wiles (at Princeton), chose to announce his proof at Cambridge, if only you heard him saying why (in Simon Singh's documentary), you wouldn't want to leave the mecca for mathematics you are currently in.