Right, well - I'm quite well qualified to respond to this thread I feel - as I've been to uni in Cardiff but am now off to uni in York.
In Cardiff I found it was all brilliant for the first few weeks - everybody was sociable and I was living in the centre of a really young person's kind of city (lots of nightclubs, vibrancy, quite beautiful, culturally interesting, loads of student nights, shopping, societies and an easily accessible Lidl
). But it started to go downhill from there. There isn't really a sense of community here. The university is big and the common areas are shared between too many or are non-existant. I found the teaching to be poor at times (although that's specific to Law) - but the most widespread problem is student apathy. For some reason there's a culture here that working isn't cool. Last week's Gair Rhydd (Cardiff's student newspaper) had a big article about it - and it's extremely noticeable. I've been to lectures at other universities and people seem much more interested/actually attend. Anecdotal evidence suggests that in seminars people make more of an effort to talk and so on at other similarly ranked universities. I don't really understand why - but it certainly seems to be the case.
York is a smaller university, has the college system, and better/more communal areas for us student kids - which would suggest that there is likely to be a better community feel to the place. The teaching is rated as amongst the highest in the land, and the students that I do know there say apathy is not as much of an issue. The city itself is nowhere near as vibrant as Cardiff, and the campus nature of the university means you're more likely to feel that you're living in a bit of a bubble. However, there are still clubs and pubs and so on, and loads of brilliant societies, from what I've heard.
In summary, I'd say choose based on which kind of place you'd prefer to live - anonymity with city vibrancy, or community spirit with quieter city. My experiences will have biased my perception - and many people love being at Cardiff Uni. However - do take into consideration the student apathy aspect - I'm not just basing that on my own experiences, after all.
P.S. Apologies if my post is a bit incoherent. Sleep deprivation is to blame.