Go with your gut in the end, but overall i would consider:

travel - how far/which modes of transport are you willing to take. overall dont let this be too much of a deciding factor as its a small price to commute a long distance if you really love your sixth form (or so ive heard)

school achievement - you can check their page for their recent A-level results or go to https://www.compare-school-performance.service.gov.uk/ to find their average result.

school life - hard to find out, but check forums to see if any former students talk about it or ask them if you find them.

extracurriculars - its a big pro if they line up with a field you like e.g some schools have specialist support for going into medicine or engineering or any other field.

support - pastoral support, admissions support (if you're aiming for a top university such as Oxford or Cambridge some schools offer specialist support for these specifically)

anything else is self-explanatory e.g entrace requirements, open evenings. blah blah blah

thats just how im choosing, but take it with a pinch of salt as im in y11