IMO sixth forms are very similar to still be being at secondary school whereas at college, you are much more independent (even though the teachers and staff may still be great). At my sixth form, we were told that we are now adults and have to be much more independent, yet whenever we don't turn up to a lesson or if we are late, sixth form send texts/emails to our parents. Sixth forms can also be quite cliquey which can be a good thing or a bad thing depending on if you have a good group of friends or not.
I don't have any experience of being at a college but I'd say compare the courses at each of the institutions. Also, if the sixth form is part of your secondary school and you'd like a fresh start, go to the college.