I don't know very much about either university. Both have a good reputation in maths. Warwick uni is based a large campus just outside Coventry, which is where I grew up. Coventry is ok, it's a medium sized industrial city, and it's close to Birmingham and reasonably close to London where there's a bit more going on. It's not very attractive though, and a few years ago at least the city centre had a reputation for being quite violent. I can't think of anything other than the uni(s) that would attract students to the city. I'm less familiar with Durham, though I have been there a few times as I have cousins living there. I believe the uni is college based, which will give a different feel to the place. The city is much smaller, and also much more attractive than Coventry which suffered from bombing during the second word war and ugly development projects afterwards. Durham has easier access to countryside, though it's also further north so the weather will likely be more rugged.