I visited both, I hated Durham and liked Bristol, its a very personal thing.
In Bristol the tutors were engaged and on the ball, and in Durham the ones we spoke to barely had a grasp on what IB is! We sat in on a lecture at Durham and even though I speak several languages I couldn't understand a word the bloke said, everyone else was looking around shrugging too.
I looked around a few of the Durham colleges, some were fab (Butler for example) but some were dire (especially Trevelyan) and as you cannot be 100% sure of where you will be put its a huge risk, I know its only a year living in but there is no way I would live somewhere like that! In Bristol and all the other places I saw there was a difference between the best and worst accommodation, but nothing else that I physically couldnt live in.