Having been round both unis, I personally preferred Durham. They have a better reputation for my course (Natural Sciences), a nicer 'feel' with the collages and the way the uni basically takes up the whole 'city', whereas Lancaster's campus was a bit crowded, is surrounded by two major roads, and didn't have quite as good a reputation. I'm not sure what you mean about the accommodation, personally I thought the accommodation at Durham was much nicer, more affordable (with the whole fixed rate, all inclusive of meals etc. thing), and had a much nicer atmosphere due to the collages. Lancaster's 'collegiate' system is basically just normal halls, they don't guarantee accommodation, it was more expensive, and you had to share kitchens/ bathrooms with far more people. It probably depends on the Durham collage though...
Was there much difference in the courses? Another key factor for me was the rather silly structure of the course at Lancaster, whereas Durham's was more logical, allowed some specialism, and allowed me to study more first year modules.