It depends what you are comparing it to. Compared to London, Southampton is cheaper. Compared to Cardiff it is probably more expensive.
Cardiff is ag reat city, probably better than Southampton (although I go to Southampton). But Southampton has lots of great areas nearby, and is close to London if that is important to you.
I don't know enough about the Cardiff course to comment but our course has recently been restructured and is apparently a lot stronger than it previously was.