If you go with an NCF plug, you will gain more resolution and more depth/space in the sound stage. As for plating, the FI-28R, FI-48 NCFR and FI-50 NCFR all have the same thickness rhodium plating on the same OFC conductor - same conductor part. While the FI-11-N1R has a phosphor bronze conductor which has an altogether different sound to copper.
Difference in sound of the three copper plugs really boils down to how damped the shells are and whether the plug has NCF or not. To me, I find the FI-48 NCFR to be a little over-damped and doesn't quite deliver the detail or extension of the FI-50 NCF.
If you want try both NCF and the rhodium plated OFC conductor and don't want to spend too much, there is also the FI-C15 NCF - best bang for the buck.
That sounds promising, I think I will get an FI-50 NCF (R) and compare it with an existing FI-28 (R). Maybe changing to the '50 is a way forward for me.
Incidentally I can relate to what you wrote about the FI-48. I'm currently testing the gold version of this plug, and compared to the FI-28 (G) it certainly does give a somewhat better resolution and wider sound stage but seems a little too laid back at the top end. So this over-dampened top end seems to be a property of the metal housing for some reason. Quite interesting because from gut feeling I would have associated a higher damping with a plastic housing.