Congratulations on your offer by the way!
I would firm Durham, simply because it has great national reputation and good international reputation so, if you were thinking of staying in the UK after your degree, this will put you in a good position for finding work. Nevertheless, Newcastle and Lancaster are also good Uni's that are slightly less reputable than Durham.
In terms of lifestyle and the overall friendliness of the place it is a beautiful traditional city that is not far from Newcastle in case you wanted a better nightlife or get bored with Durham city. I personally like it, it's quiet, but you can distract yourself easily by taking a train up to Newcastle, which is cheap and doesn't take much time.