I can only share a personal experience with FI-46 (Gold) NCF and FI-50 Gold on Oyaide Black Mamba V2 cable, since I've done a lot of comparative listening in the last month, or two.
I'm not sure if one can extrapolate that to FI-48(R) NCF vs FI-50 (R) NCF.What I hear , is FI-46 G is somewhat brighter, than FI-50 counterpart.
When i say "brighter" , I really don't mean anything objectionable, just different.
I also suspect, that a fiberglass body of the FI-50 provides better vibration control, than the metal one of the FI-46/48, or just more damped perhaps.