I think a good argument for both has been put across. I often wear a foam LJ as it only slightly restricts movement, protects the upper body and helps keep you warm. It also has pockets for gear knife, personal flares etc.
Just a couple of recomendations if your going out to buy a man/auto inflation LJ.
1- Dont forget to test it now and again, manually inflate it and leave it over night, check the gas bottle and perhaps tepe on a spare.
2- Add some reflective tape, when your strobe light runs out it is very very effective if someone is looking for you. Highly recomended by the RNLI.
3- Get a spray hood, not many RIBsters carry liferafts so if you go in the ogin and cant get back to the boat a spray hood will greatly enhance your chances of survival.
4- Make sure its got a crotch strap otherwise they often just pop over your head.
If you have a weekend spare think about doing your sea survival RYA course, only £50 odd, its one day, interesting, good fun and you get to play in life rafts.