Leupolds are great glass but they are keen on keeping their scopes lightweight, as a lot of hunters want that. However, it is a simple matter of physics -- you cannot build a VERY STOUT tube when you cut them as thin-walled as Leupold does to keep weight down. NF says **** the weight and builds their tubes much thicker, to provide maximum durability. You pay for that in weight, but sometimes the insurance is worth it, like on a multi-thousand dollar hunt. Sure the Leupold will probably hold up through many hunts, but it is not NEARLY as likely to hold up as a NF is.
NF has a video where they sight in a scope, take it off the rifle and pound the crap out of a wood bench with it, then put it back on. Amazingly, it shoots to the same spot. I'd like to see someone try that with a Leupold!
You periodically see threads in optics forums about Leupold's disappointing quality -- and they always have dozens of people complaining their Lupy wouldn't track right, didn't hold up, etc. You NEVER see a group of people complaining their NF had defects.
You wanna find out which scope is the most durable? Ask the guys shooting BMG's. They do not even bother to put anything other than NF on one of those, because they will quickly quit working.
Also, unless you are talking about Leupold's HD glass, there is no way a VX5 or 6 is as clear as an NXS. Even my Trijicons are sharper.