Probably nottingham because I live there, I have never been to Cardiff so I can't say you should go there. Only places to avoid is, Clifton, Stapleford, Wollaton and Beeston Rylands. The rest is ok and Nottingham uni is good and have tons of courses, loads of cheap flats near campus and the bus service is brill! Loads of places to go in the mean time and near to shopping centre, pubs clubs, libraries, bank ad advice centres. Good luck choosing!