Hi there, I'm a fresher and I had exactly the same struggle choosing between Sheffield and Nottingham. I'm a mature student, so if you want to take advice from an oldie then read on...
In the end I chose Nottingham as it was closer to home and I liked the look of the campus based lifestyle, despite Sheffield impressing me alot. Some things you might like to consider though, do you mind going catered instead of self catered? Annoyingly all the halls on the main campus (University Park) are catered, so I opted to live in Broadgate Park (just off the main campus), since I cook all my food. Broadgate is more like flats than halls, which seemed fine at the time, however this could affect the social side of things. The main halls look far more sociable if thats what you're looking for, the blocks seem to intergrate far better due to the way they are layed out. On Broadgate it's flats within blocks, which means knocking on flat doors and then getting to know the people inside the flat. Hence why there only seems to be a several really sociable blocks here where people have made the effort. I had visions of lots of kitchen parties as this is what it is like at UEA when I have visited, but I guess my expectations have let me down. The bar here is quiet too, which is dissapointing since Broadgate has 2000 residents. The bar on the main campus (Mooch) is loads better and has it's own small club. Things are picking up on Broadgate though, it's just a bit of a slow start. It's more like a villiage, and it just takes time to get to know everyone. But if you want to maximise you're social opportunities from the start then it looks like you'll have to go catered. Be warned though, I have heard lots of people complaining about the food!
Also, the students here can be quite rowdy, especially the first few weeks. There is lots of chanting and banter which is fun when you're drunk. But this may not be your thing. The banter creates a stong bond with the people within their hall, but also means things can get a bit pretentious. I take it as tongue and cheek, but some people seem to take it a step further, sometimes it's not nice to be snubbed from which hall you're in when all you're trying to do is make friendly conversation. Perhaps this is the old git in me not feeling the love, but you learn to ignore it quickly. However, I hear it can be like this at many Uni's but apparantly Nottingham has a reputation for being especially cliquey. But maybe thats part of its charm, and I'm sure you'll appreciate it in whichever hall you're in, because your hall is the best, right?
If you want to know about clubs, then Nottingham is full of them. Oceana is good, very big, lots of different rooms, and great on a Monday. Ocean is my favorite, it's smaller, and great on a Tuesday. Isis is another good club, great on a Wednesday. There's Gatecrasher which I haven't been to yet but is meant to be good on a Thursday I think, and Rock City, only been there once but it was great. All have exclusive student nights, the nightlife here is awesome you'll have no problem having fun. One thing to consider though, apparantly Sheffield has the best student club, but I havn't met anyone who's been there. There are also the Rag Raids here every Saturday, it's a charity fundraising thing, you goto a different city and get horribly drunk in the process. Good fun.
From what I can gather, Sheffield seems to have a more friendly vibe, and in hindsight it might have been more on my wavelength as I'm from a friendly city myself. But I guess I'll never know (unless I do the unthinkable and change). I don't think it matters which Uni is more prestigious, I think it's better to enjoy your degree. I thought I'd enjoy my degree more at Nottingham, I was wowed by the beautiful campus Nottingham had to offer, the nicer lecture theatres, the first class facilites and all the great things I have heard about the nightlife. Sheffield felt too close to home. I guess Nottingham mostly hasnt let me down, just the slow start in my halls, but thats mainly down to luck at the end of the day. I have no doubts my 2nd year will be awesome though. If you want to know more PM me for my MSN. Good luck deciding.