It isn't really fair to compare a PST and NXS in my opinion. The manufacturing of the NXS in terms or reliability, durability, and consistency of product are leaps above the PST. Better resolution, clarity, color and contrast in glass in the NXS. The zero stop in the Nightforce is a mechanical zero as opposed to the shim system that the PST works on.
While the PST is a great buy for the money, I don't think that it was developed to compete in that NXS price range, but offers a good alternative for people that can't fork out the cash on a Nightforce.
In my humble opinion the Nightforce is definitely a considerable upgrade.
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