If I understand the way the waist pack models work you still wind up with an inflatable pillow around your neck and comming down your chest ... it just takes an extra step of you putting the loop in the pillow over your head manually after it has been inflated. The extra step might be a problem in you were being swept down stream by a strong current because you'd need to free at least on hand to position the pillow over your head.

Personally, I prefer the behind the neck type because it's just one less thing to worry about if an emergency arises. I wear a lanyard and SOSpenders with pockets (room for flyboxes and other stuff) and don't even realize I have them on most of the time. It allows me to do away with a vest completely.

Dean