The University of Southampton is ranked 80th worldwide and 12th in the UK, so it is hardly even an 'ok' university. For the course I am currently studying, Modern Languages, it is ranked 3rd behind Oxford and Cambridge, and it is top 10 for engineering and medicine.
Naturally it does not excel in every subject area, but it is renound for the calibre of postgraduate research it hosts.
Southampton Solent on the other hand, is a new university, and is good for newer more vocational subjects, and is coincidentally just down the road. There's some kind of rivalry between the two that tbh I haven't quite figured out...
Mentioning AISEC my friend is the president of it here and ja, if anything, Southampton University does give a lot of opportunities and has a lot of connections. The student services and union are also OUTSTANDINGLY supportive so student life here is really excellent.