Not going to indulge some of the more blatant misconceptions in this thread but I will say this, both Manchester and Imperial are Universities placed near the absolute pinnacle of Higher Education without question.
Manchester has exceptional Biology facilities and very good job prospects for the course, especially given that the University works very closely with GlaxoSmithKline to produce the next generation of researchers and scientists in the subject, they pump so much funding into research intensive projects it is unreal and I would strongly recommend Manchester to any PhD student. This makes it a very strong pick for this particular course, and the poster prattling on about prestige needs to do some research into real world links that most modern Russell Groups have and that significantly benefit bright graduates. Infact, the real reason Manchester University isn't higher in Uni Rankings is because many undergrads get caught up in the atmosphere and culture of the city that they end up receiving the hard end of the stick from the University, lowering their satisfaction rating.
Imperial however is the creme de la creme, especially in terms of name. It trades blows with UCL and LSE for the best overall London University but it has an unquestionably strong faculty. It lacks the practical links with big companies that Manchester has, but arguably makes up for it with just its name. It also receives a massive chunk of funding, though its mainly directed at its medical courses.
Both London and Manchester are brilliant and colourful cities, I'd say Manchester is harder, streetwise and more open-minded, London is metropolitan and a bit more sophisticated I think, but thats just based on my personal stereotypes formed from spending time at both Universities.
Where do you think you're going to have more fun? At the end of the day, a first from either Uni is still extremely good and will put you well above the vast majority of graduates. You might prize that lucrative relationship with GSK that Manc has, or you might have fallen in love with the brilliance that is Imperial.
Good luck!