I had the same dilema, but between Durham, Bristol and St Andrews. All are really good unis and the departments are good in different ways. After visiting all again I've chosen Durham as I liked the college system- it felt friendlier than halls. Also I couldn't see myself living in Bristol and St Andrews for me was a little too remote. Also as the largest department in the country, Durham had quite a few different research interests so they had a really interesting list of optional modules (not saying ofcourse Bristol and St Andrews didn't either).
Also if your deferred and looking to get AAA, why not apply?
Hope that helped a bit- which college are u for Durham?