Do some research of the schools in the local area.
Have a look at the top grades, the improvement in grades from GCSE to A Level. These will just give you a bit of an idea.
The only main difference I can think of is the environment.
A sixth form tends to be located in a school along with pupils aged 10-16. It'll likely be a lot more relaxed. There'll be fewer people your age, it'll be a lot closer relationship-wise, and basically what you're used to.
Colleges are usually just people aged 16+. There's loads of people, some of whom you'll probably never speak to/see. It's more like going to work in a way. It's more down to you. It's an experience. Some love it, some hate it.
I have a friend who went from my school after his January (AS) exams to a college. He hated it and he ended up coming back. Another friend has been at college since the start of her AS levels, and she's loved it.
No one here will be able to tell you what you'll prefer or where you'll get better grades. It's down to you to do your research and make your decision.