I've heard a lot of great things about the River and I've owned a BioGold River for some time but never really got on with it. However, I have a much better time with the Project Grip River, it feels much easier to grip and throw (although I think some of the marketing as to how grippy it is might be somewhat overzealous) and the rim feels much firmer. For me the Project Grip plastic also seems to have more glide than the BioGold plastic but that could just be as a result of throwing it better. I'm not sure it's worth the price premium though, I suspect most people would be fine with Opto or Gold plastic.
The biggest downside for me is that the stamp and colours are rather boring and not very different from the other Project Grip plastic discs so they all look a bit similar in the bag.