Well I'm an Engineering prospect but I have heard from friends that Royal Holloway is a particularly good uni for Maths.
I guess, i hate to be so uselessly unhelpful, that it comes down to how much you don't want to leave London for the sake of a new university.
Personally I would suggest also that the cost of living is less in Sheffield unless you would be living at home. And if you are going to be living at home if you went to Royal Holloway then that could be a contributing factor in your decision.
Either way, I'm sure you will enjoy it wherever you go, uni is about the experience and as long as you like both places I'm sure you'll have fun wherever!