Purely on local Id go with Swansea, but I grew up in the north so I'm somewhat bias, take this into account in my post. The city itself might not be the best (I have never been myself) but I went to the uni on an open day and its pretty good. Also its in good walking country, within travelling distances of some nice national parks and some great beaches.
Bradford is a pretty heavy centre of immigration and racial tensions "can" be pretty high. Its a long way from the coast but your nearest national park is probably the the Yorkshire Dales and its very close.
City wise Bradford is nice and close to Sheffield (1hr30 train, 1 change in Leeds) which is a pretty nice place to go and visit for shopping and just to hang about, it has some lovely water features and a like, as well as having the Sheffield Hallam Arena, which every major band/show going visits when going round the north of England, as well as the Sheffield Octagon for smaller venues. Bradford has a regular trains to London with one change in Leeds (19 min run). Leeds will provide you will the comforts of a city such as cinemas and a like but in itself isnt much to write home about.
Im not in a good position to comment on Cardiff/Swansea as a city having never been. However there is a through train from Swansea to London on a semi regular basis. Its an hour run from Cardiff which should have some fairly sizeable shopping centres and an arena for concerts, as well as being 1hr30 from Bristol with you should go see at some point if you go to Swansea. Its a lovely city with some amazing buildings and ships with attractions such as @bristol, an interactive science centre.
Hope that helps.
