I'm not really familiar with sixth form but from what I googled about it, it sounds comparable to junior college or vo-tech. I have an applied associate's degree from a junior college and a bachelor's from a university. I've also taught vo-tech classes.
The main difference I noticed is that in the junior college program the focus was much more technical and aimed at learning the software and skills needed to get a job. The focus at the university level was much more artistic in nature, and aimed more at creative expression. If your only goal is to get a job, go for sixth form, and (I think - like I said not that familiar with sixth form) you could go to a university later if you want. If you want to be a creative artist and particularly if you want to go on to a graduate program and be a professor, then university is for you.
A problem I had though, when going from junior college to university, is that a lot of my classes wouldn't transfer, but I was a lot more skilled at the software and technical aspects than the students who had gone directly to university because technical competency wasn't as much of a focus in the university classes.
