Hi,
It's quite hard to give an answer which isn't 'it depends'. I think the great thing about post-16 is that you have a much better chance of finding provision to suit the individual.
So, for example, DD is a confident and fairly extrovert high-achiever who is hoping to do medicine at university. For her, she wanted somewhere with the chance to make new friends, have lots of opportunities in terms of trips, enrichment etc and also support and experience with medical school applications. She's at a large 6th form college doing the IB.
DN1 is very bright but has had difficulties with exam-related anxiety and isn't especially self-confident. She has ended up at a large school sixth form (not her original school), with really good pastoral support and excellent music facilities (her passion). It feels like a good fit for her, but actually she ended up there mainly because the 6th form college couldn't guarantee she'd be able to do one of her subjects.
DN2 is smart, but not especially driven. He's a creature of habit and likes to stay within his comfort zone. He's thriving at the very small sixth form attached to the school they all attended - I think there are around 30 kids in his year, including his group of close friends.
What I'm trying to convey with that, is that what works is going to depend on finding a good fit. So, the 'feel' is important. I'd also want to know about the flexibility to switch subjects in the first few weeks, pastoral support, class sizes, support with university applications and support with alternatives to university. How much independent study you're expected to do, and how you're supported to transition to that way of working. Also, if it's a school sixth form, what the shape of it is - is it mostly people who've been there for years, or mostly new intake, or a mix. Enrichment/extracurricular opportunities (again this is likely to be individual). How long the day is and what the options are for free periods (if you do 3 A Levels there will be quite a few). Transport - costs/frequency/options if you don't want to go in first thing or have an early finish.
Staff recruitment/retention is important at the moment too - I think especially if doing STEM subjects, but possibly applies to other things too.
I'm sure there's more...but that's some initial thoughts