You have not actually mentioned the course in your post.
Do you just 'want to go to Uni' - or do you actually want to study Physics?
You need to look at the course at each of these two Unis - in detail, because they wont be exactly the same. Which optional units would you choose (and why), what extras are available at each uni as part of the course like study abroad, units in other subjects, industrial placements etc etc. Its the course you will be studying every day for 3 or 4 years - it doesn't matter which city has the best social life if you are bored sideways by the course you are doing.
Bristol as a whole is generally considered better than Manchester as a whole.
Although I've heard some international students worship a Manchester degree.