Pretty sure FF+ is basically the same material as Dynaflight '97--Dyneema 75. Original FF was Dyneema 65--same material, just a lower grade.
For what it's worth (I've made quite a few strings, but I'm no pro shooter by any stretch), the main differences I've found in shooting is how the string is made and the number of strands. I use/prefer Dynaflight '97 because it seems to be a tad more durable than 450+, and doesn't require as many strands for the same size string as 8125. I wouldn't have a problem using any of those three.
I have talked to a few folks whose bows seemed to "like" one material better than another, but if anyone has figured out why I haven't heard about it, and it's a rare occurance as best I can tell.
IMO, a properly made and properly tuned string makes a lot more difference than what material it's made from, at least concerning high performance materials and how they shoot.
That being said, I never cared for original FF, and after talking with a Brownell customer service rep some years ago, it's not likely I'll ever use anything other than BCY products. Great people to deal with, always treated me like I was a million dollar customer--I'll pay a couple bucks more per spool for that kind of service vs. being talked to like a red-headed step-child.
Chad