Choosing a sixth form Watch
Often the figures are misleading anyway. Sixth forms kick out anyone who performs poorly at AS. This means that only the people who are likely to pass are allowed to go to A2 anyway.
Choose based on where you feel happiest. You'll be spending at least two years at the place, so make it somewhere you'll not mind going. Go somewhere that does all the courses you want to do.
The only reason not to pick a sixth form is on the basis that 'all my friends are going there'.You go where you think you are likeliest to do well.