When I was talking to the admissions tutor at QMUL, I was talking to him about my choice of having Sheffield as a Firm. He said don't, but then said if it was Manchester or somewhere, he would have said that would have been a hard decision.
QMUL is in London, which is obvious the Capital in many ways! Its definatly not in the most expensive part of LDN, if thats what you're worried about, and Mile End is not what everyone says it to be. In terms of rep, I'd say QMUL was better. Looking at league tables (so take it with a pinch of salt), QMUL is around 12th, not sure what Manchester is.
At the end of the day, rep is one thing, but you should be going where you WANT to be at. Go with you're gut instinct!!