Ime college sixth forms allow more freedom. My dc both stayed at their school, with dress code (tho not all schools have this) and they were not allowed to leave the site.
Their peers at the local college could wear what they liked and leave the site if they had frees - handy for driving lessons.
Otoh ime the school sixth form was better for support with UCAS forms and PS as there was more focus on dc heading to uni. Tjo that did mean that if your dc wasn't planning on uni, they got put under a but of pressure. But that may just be that school.
Colleges are more likely to offer other qualifications such as BTECs and T levels. School will usually focus on A levels.
Personally I would have advised my own dc (looking at traditional subjects fir A levels and a degree- eng lit, French, biology, maths) to stay at a school; but there are certainly arguments in favour of college. What does your dc want to study post-16 op?
Btw some places, like Winchester, only have sixth form colleges as an option, so the above jnfo may well not apply jn the same way. And of course I am generalising- plenty of good teaching of A level Eng lit and French at colleges and some schools do offer the odd BTEC.