I've not been a student in the cities mentioned, but I've lived in Sheffield, spent a fair bit of time in both Manchester and Bristol and a little bit of time in Nottingham. I find Sheffield has the most laid back vibe of them all. My son is actually going to be studying at Sheffied Hallam from 2017 and I feel very relaxed about it. Manchester has more of a big city feel (like a small London), but has a terrible reputation for crime historically. When I lived in the north in the 1990's it was nicknamed 'Gunchester'. Bristol is a lovely city but still feels 'edgier' to me than Sheffield ever did. Nottingham feels like a big small town rather than a city and seems pretty rough at night from the few times I went out with friends there. Hope this helps.