A bouyancy aid is not meant to make you float, it's meant to aid your bouyancy. Hence the name. They are intended to allow easy movement and enable swimming. They are suited to activities where you may be in the water on repreated occasions.
IMHO they are designed for different purposes and are generally not interchangeable depending on personal preference.
Scoops
Bouyancy = floating in the water-not sinking. Bouyancy aids are rated for the weight of the user. The heavier the user the more bouyancy required for the user to float more easily. People are naturally bouyant and even more so when treading water etc but a bouyancy aid allows the person to conserve energy and float easier than without.
I'm a big guy and I've yet to find a bouyancy aid anywhere with enough bouyancy to be of any use to me. In fact they have proved more of a hinderance in and out of the water, constrictive, uncomfortable etc hence the use of a life jacket.
As someone who rarely intends to have repeated swims, sometimes crosses open water, even in the sea, with a canoe or touring kayak; for me a gas inflatable life jacket gives me the security that I require with the comfort that I need to when paddling. Sure it breaks away from accepted practice within the BCU etc but worldwide this is not unusual. I can easily get back into my kayak wearing the life jacket and the canoe and have the option of inflating it if and when the need arises. It's horses for courses at the end of the day.