I start all my horses in the snaffle. When they have been in the snaffle long enough that they aren't getting any better with it, I'll move into the bosal. Usually I'll stay in the snaffle for about 1 1/2 to 2 years. When I go to the bosal I'll do it gradually, switching back and forth between it and the snaffle for a few weeks or months depending on the horse. At this point the horse should have enough savvy to handle the bosal. Kevin is right. Experience is necessary to use the bosal effectively. About the only thing I accomplished with the first couple of horses I used the bosal on was to teach them to run through it!
My suggestion is to stick with the snaffle.