The NSX is nicer in many ways to the Mark 4s ( the more common Tacticalized VX-3 ...the original MK4 fixed power are very different form the new style variables...but in all fairness more expensive as well )
First, you have a zero stop option. The only zero stop you get with the Mark 4 is when you use a M3 type turret but then the adjustments are far more coarse
The clicks are more positive in the NSX ( nothing like a S&B PM II however ) than the Leupold M1 turrets. The side focus has less lash as well
Generally the NSX has significantly more elevation adjustment 110MOA vs 65MOA. The only exception is the real original M4 fixed 16 Power that offers a zillion MOA. You may not need that much but at least its there
More recticle choices. Option to have Miliradian turret with matching MLR recticle is pretty nice. Both have FFP versions so thats a wash
Its easier to adjust illumination intensity with Mark 4 but having said that I never play with the intensity of my NSXs
I have lots of Leupolds but the price of the Mark 4s are creeping up to what a NSX is. I think for the money there are better choices out there ( not only the NSX but also the IORs )
In reality however, both scopes are far better than most of the typical scope you see at the range / field