I own one MK 4 6-20x50 and two NF 5.5-22x56s and there are very few advantages to either. After having both scopes at the range I decided to take a closer look for comparison.
My findings were minor to say the least. The glass is very good on both in fact it was inconclusive to me which one was better. They both track very well and return to zero perfectly. The edge went to Nightforce in terms of adjustments, the clicks were slightly more audible and more crisp(less creep). The leupold is more compact and quite a bit lighter. I like the Nightforce mildots better because they are more of circles that you can see through rather than a solid dot. The Nightforce MLR reticle is better than both of them.
My conclusion was as a range rifle, I would put the Nightforce on it, but for a field rifle, especially hunting, I would go for the Leupold.