OK I have 4 IOR and another on its way. IOR is what I now choose for applications where I cant justify an S&B PM2
1) IOR use Schott Glass - as used by USOptics and others - The coatings were (I'm unsure what the current stand is) Zeiss Tessar (these were originally Camera lens coatings and are not supposedly colour biased). Very good glass and excellent coatings.
The Zeiss Conquest is nowhere near as good optically (it is NOT made by Zeiss either). The newer Zeiss Victory (diavari) are just better imo - older ones can be a bit "washed out". I'm not a fan of Nightforce (they're SFP) so have not got one to do a decent test with. S&B much better. Leupold hurt my eyes so I don't have one except a hybrid that has an S&B objective lens.
2) Most IOR are SFP but IOR understand the need for FFP (esp in Europe) - They have some FFP hunting scopes and the 2.5-10*42 FFP as well as many fixed mag. The 3-18*42 is now generally available in FFP too - The Snipershide scope I am told by the factory is different though. The rationale of a FFP in a factor 6 zoom - I'm yet to be convinced - but so far not seen a scope much above factor 3,5 that has it cracked. There is a solution and afaik only USoptics and IOR have even considered it. Neither has afaik done much work on the idea yet.
3) IOR scopes CAN be heavy. This is imo their main downside. The S&B 3-12*50 PM2 without PX is lighter than the IOR2.5-10*42. Only just but... There is however an "interesting" price difference.
4) BEWARE the "bright" trap. If I could get hold of a spectrometer I'd really go for this. BUT my eyes are hypersensitive to blue light and many of the so called "bright" scopes are simply passing a lot of blue spectrum (high energy) light. This is NOT necessarily a good thing (in my case it makes my eyes hurt) - It's more or less accepted I think that blueblocker type (UV block) sunglasses are good for your eyes - think about it!.
ime IOR scopes offer good contrast and the reticle is visible against the target image in all conditions. S&B only claim 90% transmission for their PM2 line. What's more they tell you the wavelength range. So beware the transmission numbers.
What are IOR are not, is "pretty". Also they do something I like but which sort of affects them in the "fashion" bit. They paint the outside of their scopes. Not as pretty but they don't reflect IR light.