Something to consider is that as I understand for Durham some colleges (like University College aka Castle I believe) have shared rooms for first year (i.e. US style where there are two beds in a single room on opposite sides, with two students sharing the whole room with one bed each; this was the case for a friend of my sister's who was at Castle albeit some time ago). This may not be appealing for everyone so could be a major consideration/something to investigate before making any choices!
I also gather in general a disproportional number of students put Castle as their first preference college and end up getting placed in their second or third choice as a result. Although obviously if you want to have a chance at Castle, you would need to put that as your first preference I believe. Unlike Oxbridge I don't believe the colleges themselves make decisions on admissions and I believe academic teaching is even more centralised, and you just put a ranked preference of colleges rather than "applying" to a single college in the same way as for Oxbridge. So I think overall don't worry so much about college choice if you like the university in general and just pick a "best fit" option?