Life jackets are designed to keep you afloat in water. They will automatically turn you into a face-up position, keeping your mouth and nostrils clear of the water to prevent the risk of drowning.
Buoyancy aids are simply that - an aid to buoyancy that generally relies on help being close at hand. It assumes that the wearer is able to help themselves to some degree by swimming to safety or by keeping themselves afloat while assistance arrives if required.
Flotation suits and vests will protect from the cold and can offer buoyancy in addition to your lifejacket. However, they are not designed to turn an unconscious person face-up in the water and may be too loose to provide proper support, so are not recommended as a primary aid.