Not sure what your intended use is but A life jacket is just that a “Life” jacket. If you are using it for flat-water (lake or flat river) a inflatable is just fine, but any moving water over low class II (now your back to flat-water) is ill advised in my opinion and in the opinion of most all people who have any river experience. A inflatable jacket can rupture, uncomfortable to paddle in when it is inflated so you may be tempted to just inflate it when you need it (that would be after you are in the water gasping for air), inflatable life jackets are not made to swim with, pretty much impossible to maneuver yourself in the river to avoid obstacles unless your on your back, your just a bobber floating down the river. The idea of floating on your back with your feet out in front at all times through a rapid and actually getting to shore is a misconception, sometimes it is a better idea to roll over off your back and “swim like hell” to avoid an obsticle. If it happend, and it does, getting under water to lets say swim through the strainer you just got swept into may be difficult with an inflatable jacket (too much flotation) your chances of puncturing are higher and not a place you want to loose all you flotation. Oh and one other thing in regards to inflatable life jackets and moving water (class II-II+ and beyond), people with vast boating experience will think you are a liability and not want to paddle with you not to mention what they say about you behind your back. The same thing goes with lets say wearing a “bike” helmet to boat with, this could be a entirely different post on why this is unsafe. But like I said before if you are only using it in flat water an inflatable jacket is useful and super lightweight so this is not meant to be critical of inflatable jackets, they have a place to be used and a place they are inappropriate in my opinion.
On the other side of your question finding a lightweight jacket if your using it for moving water, try looking at a pullover or side zip kayaking type III life jacket. They only have two slabs of foam and if you can find one without all the bells and whistles (excessive pockets, zippers, attachment points) that will also make the jacket lighter, but the fact is that to get 15.5-16.5lbs of flotation at sea level (industry standard for type III life jacket) is going to weigh a minimum amount with the weight of the foam and shell holding it, try to get rid of the extras you do not need (once again get rid of excessive pockets, zippers, attachment points). Astral, Kokatat, MTI, Stohlquist, all make what they call "playboating life jackets which are slim cut for paddling and have very few bells and whistles and minimal foam to actually float you but allow you to swim also, but by the way you should actually have a whistle you just don`t need a zippered pockets to put it in.
Hope this helps some.