An additional consideration if you are operating on big water, at high speed, or near rocks, etc.: A nice layer of thick, bouyant, closed cell foam also provides you with some degree of impact protection.

Hit something hard with a traditional vest and the foam provides you with some protection. Hit something hard with an inflatable, and you may rupture the bladder.

That said, they are MUCH more comfortable, and I use both regularly, depending on where I am going, what kind of vessel I'm on, and what the conditions are.