Staying for Sixth Form was the worst choice I could have made. The past two years have been painful- simply because everyone knows each other, cliques have been formed, and I'd much rather have left and started fresh. At the time I went for the easiest option, but it's become hard in the long run. I suppose it depends on your circumstances though. If a few of your friends are staying on, you might want to stay.
During Yr 11, my school hyped its Sixth Form and all its qualities, promises of adult treatment, etc. I generally find that hasn't been the case; they're used to teaching lower year pupils, a lot of them can't differenciate between how yr 7s learn and how yr 12s learn. I find that college has a much wider variety of course choices, too.