I am familiar with both Warwick and Durhams maths courses so Ill try and give you my view. Academically Warwick is regarded as being better for maths, but in reality the courses are fairly similar; both are of a similar pace covering more or less the similar material. However, at Warwick you get more assignments, most of which are for credit and are more difficult than the assignments at Durham, most of which arent for credit. Also, the exams at Warwick seem harder, but thats just from the papers Ive seen. Durham has a much smaller intake, ~110 I think whereas Warwick has an intake of ~280 if youre interested in that. And as overlife said, Warwick University is very much behind its Mathematics department which is evident from the new building.
If you value the social experience, Durham is better in my opinion.
Overall, youre fine going to either University. If youre good at maths then both will equip you well in terms of careers.