Polytechnics were more geared towards HNDs and HNCs which are vocationally based. Hatfield Polytechnic (which became the University of Hertfordshire and swallowed up the Art School I was at in the process) was heavily geared towards engineering. Just down the road was BAE, so you can imagine that it was a short walk from a qualification to work.
Degrees require a level of academic work which the HND/Cs don’t. My course started off as an HND, and it was mostly practical, with some written work - but the emphasis was definitely practical. It turned into an Honours degree at the beginning of the second year, and it started to get more academic for those of us who wanted to progress to the third year. I am wondering how that academic work, outside of personal interest, would really help in the world of graphic design. Having the degree does mean that I have a level of qualification which makes me more employable, but the HND aspect of the course is surely the better foundation.