I had the same Question a while back, except it was between the Zeiss Conquest and the IOR rather than the Nikon.
Go to this thread to find my post and pics comparing the Zeiss and IOR side by side.
I'm not exactly sure if the link to the thread will work, if it doesn't, do a search for IOR within the past year and my handle and you should be able to find it.
I love the IOR 3-18 x 42mm scope. It is clear sharp and the MP-8 reticle is a thing of beaty. The only thing I don't like is the illuminated reticle. There was no other option at the time. Nothing wrong with it, I just don't have a need for it and its another clunky thing that ruins the asthetics of the scopes fluid lines.
I have this scope on my .270Wby which is a heavy 11lb gun. At least I think its heavy. But the rifle shoots great and I love it! (Smithed by Kirby)
That being said, it looks good on a heavy long range gun but I would not put it on a lighter hunting rifle. I used this rifle to bag a deer in Wyoming last year and toting that gun around was a hassle. It was real steep country. I had the rifle slung over my shoulder, and when I bent down to get under tree limbs it would occasionally bonk me in the head. [img]/ubbthreads/images/graemlins/confused.gif[/img]
For this reason I am having a new custom rifle built. It will be a switch barrel rifle in .270WSM for deer and .300WSM for elk. It will be light weight and it will wear my Zeiss Conquest 4.5-14 x 44mm. This is a great hunting scope. I has the turrets, which are exposed, no cap, so you can dial faster and it is lighter and just looks better on a light weight rifle. The glass is sharp and clear, mechanism is repeatable and best of all it is around $400 cheaper than the IOR.
So in conclusion: Long Range Heavy barreled gun = IOR
Light Weight Hunting Rifle = Zeiss Conquest.
Look at both side by side and decide for yourself. You really can't go wrong with either.
Good luck and post pics of whichever scope you end up purchasing.