You've done very well to get into Cambridge - congratulations!
I applied to Oxford and Durham knowing I'd be delighted with either, and having been rejected from oxford, I'm nervously waiting for Durham's reply...
I think the question really hinges on where you really want to go. I see your reasons for considering Durham, and they are probably true - Durham courses are known to be intensive, but I think the 'stereotype' and therefore a lot of the pressure is removed. On the other hand, Cambridge is Cambridge. You have to consider where you will feel happiest - they're both great places to live and study. My friend turned Cambridge down for History, to go to Durham, and she has never regretted it.
My advice is until you have an offer from Durham, don't do anything, because it's not unheard of for them to reject applicants with Oxbridge offers.
Try not to stress - it really isn't going to drastically affect your life.
Well done again!