Determine your budget for optics, decide which features matter the most to you, then buy the absolute best one you can afford.
There are so many options out there today that it would be hard to make a recommendation for you. You are in the right ball park for power range in my opinion. Of the 2 you listed I would lean toward the Mark 4 just because I like exposed turrets. If you go with Nightforce just get an NXS with HS and ZS. Scope would serve you really well and last a long time. Bushnell and Vortex also make some great "value" optics...which is to say they make a nice compromise of all the features available in the world of optics and put it together really well for a good price.