I've owned a couple of both scopes. The Mark 5 definitely wins in optical performance, and the turret is also a little more crisp. That being said, I wouldn't own another one. They are not dependable and their QC is not good either. I received a MOA Mark 5 that accidentally had MIL internals. Took me a while to figure out and had to send it back twice to get them to actually repair it. My dad's Mark 5 lost zero by 2" on our last hunt after riding in the back of the ranger for 1/2 a day. We shot it the previous day to confirm zero, and then again after he hit high twice on a buck. My close buddy's Mark 5 just flat failed and had to be sent back. Along with several VX HD's going down by close friends of mine. They just simply can't be trusted IMO. I'm sure there are plenty of them out there that haven't had any issues also.
NX8 is just a solid scope. Not great at anything, but good enough at everything. And should hold zero through a lot of abuse.