There a few minor differences between colleges and sixth forms, but neither one of these options is categorically ‘better’ than the other.
Sixth forms tend to be a bit closer to the experience that you will have had at school, most commonly due to the environment surrounding them. The teachers may request that the same level of formality is retained, and may still want to be addressed as ‘sir’ or ‘miss.’
Colleges have a tendency to be a bit more relaxed, with no dress code and more of a chance to express yourself in this regard. Many college tutors are fine with, or even prefer, being addressed by their first names.
On the whole though, either choice is just as good as the other, and it really comes down to your own decision and preference. Make sure you attend open days and look around to see which you think would suit you better.