If it doesnt feel right then don't go and take a year out if you arent going to be happy. You might get better grades and that might open up options.
I think Sheffield does well in the National Student Survey, as much as they are useful. Not sure its that important whether its all campus as its a bit spread over the edge of the city centre, but covers a large area. Its a big place, but you tend to go between your department and the student union imo.
Talk to some students on both courses. You dont sound too keen on leaving home, so maybe Southhampon will be more manageable for you.