They're 3 places I'm very familiar with, and such different unis.
Warwick uni isn't in Warwick (where I live) at all, it's in Coventry (albeit on the outskirts, not in the city itself). So there is a lot of on campus accommodation. 2nd years often live in Leamington Spa, where there's a good student population, but it's very much a town, not a city. So if they hanker for city life, it may not be for them. You can easily get to Cov or B'ham, but it's not a drunken stagger back from the pub/club/gig to your accommodation. The uni has great facilities though - including a brand new sports centre which is amazing, if that's his thing. The on-campus arts centre also attracts good comedy performers, but is less well served for 'current' music acts.
Southampton is a great place to be a student (or was 15 years ago...). The uni isn't in the city centre, but it's still easy to enjoy both the campus and the city. Halls are in good locations, the campus is lovely and it's very highly regarded for engineering. There's easy access to the coast, New Forest, but also London and Portsmouth (for big-city change of scene).
Bristol (most of my family live there) is a great city. It's got such a great feel to it. Good shopping, a real buzz on the harbourside, lovely green spaces to escape to, a campus much closer to the city than the others.
I think of Bristol as a very 'manageable' city. Apart from my time at Uni, I've never lived in a city (I'm a country girl), but I've always said Bristol is the one city I would very happily move to.
None of which answers your question about how on earth you decide where to go... If you can, I'd go and visit the cities/campuses. I think you'd tell a lot about them just from a 'drive-by', particularly Warwick which I'd say is the outlier in the 3. If you do go there, be sure to drive to Leamington too.