I have 2 Mark 5's 3.6-18x44, but have only had them for a few months. I've owned NXS's, but only in the 5.5-22x50 version.
The only two things I can think of that I don't like better on the Mark 5's is that for some reason Leupold has trouble getting the little click lines to align perfectly with the zero line. It's a small thing, but it does bug me. The other thing I don't like is that the zero mark for the windage is at about 10 o'clock instead of 9 o'clock on the windage dial. In guess they did this to make it more visible while in the shooting position, but it's weird to me and I don't like it. Not a big deal though.
Everything else on the Mark 5 is better in my opinion. The glass is better. The reticle is better. The size is better (though I'm comparing it to the 5.5-22x50). The upper end magnification is way better (I couldn't get by with only 10x). The dials, zero stop, rotation indicator are all better on the Mark 5. I found the eye box and eye relief to be be a little better on the Mark 5. Magnification selection mechanism is smoother and easier to turn on the Mark 5 (especially in the cold). I like the ranges marked on the focus knob on the Mark 5 rather than just the lines that are on the NXS. Of course the NF's are all illuminated, and my Mark 5 is not. I literally never use illumination, so it's a plus to not have to pay for it on the Mark 5 if you don't want it.
In terms of durability and longevity I can't say because I haven't had the Mark 5's very long. They've been perfect so far. Pretty hard to beat a NF in this category.
Overall I've been impressed with the Mark 5's. Really nice LR hunting scope because of the size/weight, magnification range, and overall features.
