Both are very good universities. Sheffield is a city university, only 10 minutes walk from the town centre, and the buildings are on the streets rather than on a campus like York. Sheffield has an excellent students union, known as one of the very best and the city is cheap and student friendly, as you say. The accommodation is in 2 large 'student villages' which are almost brand new about 20 minutes' walk from the university in green Victorian suburbs. Everyone I know who has any connection with Sheffield (studying there or visiting friends seems to absolutely love it).
York is a beautiful very touristy town/city with an ancient Tudor centre and lots of beautiful old buildings. The campus is a few miles away from the centre and has a huge lake and lots of green areas (unlike Sheffield). The older part of the campus is 60s architecture and very typically concrete everywhere. The collegiate system means that you live in colleges/halls which were built mainly in the 60s and are therefore much older than Sheffield's accommodation, although there are some newer blocks. From what I've heard people tend to socialise more in the colleges, so the students union isn't at the centre of student life the same as in Sheffield.
I'm sure you'll love whichever you choose, although the experiences may be very different, but just make sure the offer from York is pretty much guaranteed before you pull out of your Sheffield offer. I can't advise too much on prices either.