Recently I saw a guy wanting to buy an FRV on Spearboard. He was willing to pay a good price for my Mark 2S, so I sold it to him and paid a bit more for the latest version. Now I have owned each version since the original that used a compressed air bottle that had to be filled from a 3000 psi scuba tank. They keep getting better ands better. Here are the main differences I've noticed between the Mark 2S and the Mark 3.
The fabric cover over the inflation bags seems more slippery. I don't know if that really means less drag, but it can't hurt.
The previous versions used two 38 gram CO2 cylinders while the new one uses a single 60 gram cylinder. This reduces the bulk and weight a bit as well as saving a bit of time when you are removing and installing the cylinders after each day of diving.
And here is what is probably the most significant difference. Previous versions of the vest used a rechargeable battery, which seemed like a good idea since you never had to buy new batteries. But to recharge the battery, you had to remove a threaded plug to insert the AC charger cord. And then you had to remember to replace the plug. If you didn't, the electronics flooded with salt water and ruined the FRV. There was no fixing it. Apparently quite a few people forgot to replace the plug, and I even know one of them.
The new version uses three disposable batteries. I think the manual says that they are good for about 30 eight-hour days of diving. If you were on a trip, you wouldn't have to remember to bring your charger or find an AC outlet. Just install your spare batteries. And best of all, the way you turn on the vest is to tighten the cap that holds the batteries in. Unless you forget to turn the vest on before diving, this should make it impossible to flood the battery compartment.