Do you have offers for both?
If so consider factors like location of placements, whether an intercalated year is offered (or required - and whether that's something you do or don't want to do), what co-curricular activities and facilities they offer (e.g. societies, sporting facilities etc, whatever is important to you!), average rental prices for students in each city, cost of living in each city, location of the city relative to where "home" is for you (and associated costs relating to transport if you anticipate going home on most of the holidays), stuff like that. The practical side of where you're going to spend the next 5 years of your life! As above which uni you go to won't make a difference in terms of your career if you do get in, so it's just how well your experience of that time will go (no point going somewhere you would hate for the 5 years - that's a long time to spend somewhere you don't like!).
If you haven't yet applied then you need to focus on the shortlisting methodologies of each uni and see how you will score in those - do they score GCSEs, and if so are your GCSEs competitive? If they don't score GCSEs and it's all on UCAT, do you have a competitive UCAT score relative to previous years? The other factors noted above are also a consideration but the primary consideration will be maxaimising your chance of getting an interview (and hence potentially an offer!).