I prefer the sling, it's supposed to be better for the spine and hips (of the baby, although mine benefit too). DS is a real wiggle-bum baby; he has never fallen out of the sling. He has once managed to wriggle so that he wasn't as secure in the sling as he should be (started off facing me, ended up cuddled next to my tummy), but he was still secure in the sling. He was fast asleep - when he's awake and tries to free his arms I just tuck them back into place. And he rarely tries to free himself as he loves the sling, especially when DH is wearing it. The Baby Bjorn just hurt my back and shoulders when carrying DS for any distance, the Kari-Me (UK's version of HAB) is so comfortable.

I'm sure a local friendly BB girl (and aren't they all friendly?) will let you try their sling for a couple of hours if you're worried about safety, but they really are fantastic.