I'd say an osteopath concentrate's on the muscular system as well as the skeletal. Where as a chiropractor mainly focuses on the skeletal.
In short, a chiropractor makes smaller manipulations over many appointments, where as an osteopath causes louder audible cracks, that leave you laughing nervously hoping your neck will still be attached to your body when you sit up, that said usually less appointments are needed.
I've been to both, I'd take the osteopath over the chiropractor as tight muscles can pull at bone so it is no good aligning the spine only for tight muscles to pull it out of alignment again. My osteopath never sort to make appointments just to make more money.
A physiotherapist usually works on exercises to help strengthen the muscles.