Durham is not the same as Oxford. The city is a lot smaller, for one, and a lovely one at that, but very small, with all that that entails. On any given day you are likely to run into fellow classmates as you make your way around as there are only so many places. Ditto for when you go to cafes or pubs or clubs (the few that there are) which some may find reassuring and others stifling. I'd strongly recommend coming for a visit as it's worth a day trip regardless and you'll get more of a feel for it than anyone on this forum could give you.
As others have pointed out, the collegiate system has little bearing on your studies, and instead serves as a base for your initial studies and for you to build up friendships with your fellow students.
Durham University is a top university that will demand you dedicate a considerable portion of your life to study. There is some argument to be made that Oxford and Cambridge are even more demanding however, but unless you are a super-genius I think you will find your education at Durham sufficiently rigorous.
Finally, do you have a plan for your Gap Year? Is there something that you desperately want to do, places you want to go etc? If the sole purpose of your gap year is to give you another swing at Oxbridge, I'd consider whether there's anything you can do during that year (volunteering, work experience that bears relevance to your desired course) to bolster your application to give you a better chance of success next time round. As others have pointed out, an offer from Oxbridge will never be a sure thing and you should consider whether delaying your studies for a year is worth a punt.