Durham and Exeter are effectively clones of each other just at opposite ends of the country.
Neither are very diverse - either the towns or their student populations. Lots of middle-class students from the home counties.
Both very sporty
Both small cities, so very easy to walk everywhere.
Both have limited social offerings - a couple of clubs each and lots of pubs.
The only real difference is Durhams collegiate system.
You'll have a blast at either, unless you're into clubbing.
I also wouldn't over worry about the posh aspect. Both are equal in their posh quotas - but both also have equally large numbers of normal/non-posh students