Just to answer your other points...
City economic development - I would say there's no difference, all of the UK is well developed. There is probably more unemployment in South Wales, but not that you would notice, as a university student. Public transport in Treforest/Pontypridd could be challenging (not because of lack of development, but our public transport is privatised, so on Sunday it is not profitable for them to run services).
Safety - I don't know, but neither areas are known to be dangerous.
Unfortunately, Northampton and small-town South Wales were both strong Brexit-voting areas, with an anti-immigration atmosphere. I have a French friend in Northampton who finds this tough, but more because she feels really sad, and not because she feels in physical danger. As a university student, you probably will not meet these sorts of local people. Also, Chinese students are probably more welcome than Eastern European workers.
Cool place to be - Cardiff definitely number one, it closest to being a real city. Northampton is a small town so ok. Treforest/Pontypridd is really small, but might be ok if the university bars are good.