I have shot both of these scopes many times. Here is what I see as the main differences. The PST is a great scope for the money, but it in no way an equal to the NXS and price reflects this. My biggest complaint for the 24 x PST is that while on max power it has a very fussy eye box to get in. Meaning it is hard to get and maintain a full field of view no matter where you position the scope or set your cheek rest. So I find myself turning the power down to get easier access to the full view.
I also am not a fan of the tiny numbers on the power select ring. But that would not be as big of issue with younger eyes.
If you were to go to the range with the PST alone you would probably be fine with it. Again it is a good value. But if you ever shot an NXS I doubt you would still feel the PST joy and would be looking to move up.
The NXS is an investment and a safe one if you are serious about long range. You will not out grow it soon and if you decide to sell they hold their value used pretty darn well. I would look for a used NXS. Then I doubt you would loose many $ at all even after a few years of use.
So if the budget is there, I choose the NXS.
PS: also be aware of the new offering from Nightforce for 2014. A 4~14 for around a grand. Might be just the ticket for your needs. I don't know as I have not seen one yet.
Nightforce announces new lower priced 4-14x56 SHV riflescope