I go to a sixth form and do A levels, generally sixth form is a good idea if you're planning to go to uni, whereas colleges generally offer more vocational courses e.g.BTECs and NVQs in health + social care, hair + beauty, plumbing etc so might be a good plan if you know what you want to do and want to get straight into employment. I will say sixth form is very different to being in the lower school (I stayed on at my school from yr7 to sixth form), e.g relationships with teachers are better, they're less autocratic and tend to have more time for you, relationships with those in my year outside my friendship groups are better because everyone's split up more for A levels compared to GCSEs, relationships with younger years are different, e.g. I got the opportunity to coach lower school football teams and run a mental wellbeing club for the lower school, and you can hold positions of more responsibility, e.g. we had subject ambassadors and a student senior leadership team that reflected the senior leadership team of teachers. Some sixth forms also offer AS, EPQ, BTECs and IB so worth looking into various qualifications to see what might help you get into a career you like. My sixth fro is at a grammar school and it's an academy (which means it's funded by the local authority rather than the government proper I think) so I might have a different experience to others in sixth form but can always visit a few local sixth forms and colleges to scout them out and see what you like more.