I would recommend newcastle because you wolud had the experience of living outside home, in a very different atmosphere, with only you managing your time, money and with no one tellng you what to do. It would be a great vital experience that would make you a more independent person. On the other hand, york would be a more easy and comfortable option as you would live in your city in your home with your parents and be with your "always" friends, and it would be cheaper as you wouldn't have to pay for housing. I think Newcastle would be better but york wouldn't be a bad option too.