Hey there, congratulations on getting offers from two top unis! Speaking as UCL medic with several friends at Imperial, I think if diversity in the student body is more important to you then I would choose UCL. Bear in mind that practically everyone at Imperial is a science student whereas there are loads arts and social science students at UCL. In addition, if you plan to live in halls in first year, UCL is part of the University of London (Imperial isn't) and has lots of intercollegiate halls, e.g. I lived with people from SOAS, Birkbeck, Queen Mary, King's College London etc. in my first year. However you have indicated that you may be living at home (because the proximity of Imperial is a pull factor??) in which case the halls point doesn't apply.
Another difference, as somebody else has pointed out is the fact that at UCL we do our BSc in the 3rd year whereas it's in the 4th year at Imperial. Many students I've spoken to at Imperial have said that they would've have preferred the UCL structure.
Best of luck wherever you end up :-)
Alex, 4th year UCL medic
6med