Just asked DS this question as he's the one who has actually experienced both - his answer was HOLIDAYS! (Rather a sore point as DD is still at prep school).
Other than that:
2) Cheaper Uniform
3) Really nice kids and parents
4) Less bullying
5) Much Less feedback
6) Far more Cover Lessons (which DS absolutely hates)
7) Poorer discipline
Poorly structured reward system
9) Poorer facilities
10) Higher staff turnover - a bit unsettling
11) Considerably less games and PE
12) DS spent most of Yr 7 complaining he was bored whilst waiting for kids from state schools to catch up.
However, despite there being more negatives than positives on my list, DS is very happy there and is progressing well and has finally found a nice group of like minded friends.
Would we do the same again? - we're just about to with DD.