First understand I am not going to address the FFP part of these scopes. I do dial my shots and shoot at times past 2000 yards so for this I do not want a FFP scope.
But to compare the differences between the IOR and NXS in basic function here is what I see.
Glass on both is great, so no issues there. I prefer the zero stop that NF has. It is a solid very good working system. I find the turret detents of the NXS to be a little more definite. The IOR is good but I like the feel of the NXS better. I have had one of the IOR's fail in the past. They did replace it but never the less it failed and caused me a little grief. I have never had a NXS fail and I use a ton of them. Todays IOR's are better, at least most of them. But in the past there has been some issues with them (the old blue ring model). I have lost again, the rubber eye piece rings on both IOR's. I need to call and get replacements again. This is common and I will glue the new ones on this time. They fall off easy and get lost if pulling out of a case or back pack.
Like I said, I have both and like both. These are small issues, but my trust and preference goes to Nightforce.
Jeff