Like with anything else, this depends on which exact models you are looking at.
The 3-18x42 IOR is on its fourth generation now and it looks like the bugs have been worked out. However, in terms of configuration, the new 3.5-18x50 IOR is probably a closer match to the 3.5-15x50 Nightforce.
I looked at the new 6-24x56 IOR and I really liked the new eyepiece design which it shares with the 3.5-18x50. However, we will not know how durable it is until there are some more of these in the field.
Between 3-18x42 IOR and 3.5-15x50 Nightforce, if it goes on a heavy kicker, I would lean toward the Nightforce, simply because it has more eyerelief. Otherwise, I think IOR offers a bit more for the money.