IOR glass is great, some of the best around.
I had one about 5 years ago and have been tempted to buy another. Unfortunately the model I had (3-18x FFP) failed me multiple times through several generations of that model and so I keep talking myself out of it. But then again every Leupold MK4 I have owned (and several issued) has failed me at one point or another. I'm not particularly rough on my gear, especially if I pay for it. Leupold is quick to repair or replace, but it is still a big hassle. However, if something goes wrong with an IOR you are messing with a company that is in Europe with a representative in the US. Waiting on a replacement can really be a hassle when it might take weeks to months.
But on a more positive note, I seem to have the worst luck than about anybody you'll meet. So take my experiences with that in mind. Plus most of what you hear about either of those companies is positive, with most folks being pretty satisfied.
I got my first Nightforce a little while back and if it craps out then there no hope left for me!