thinking about upgrading my hunting scope from a pst 4-16 to the nightforce. I am guessing the nightforce will be a lot more clear glass. Will I gain much in low light conditions with the nightforce?

I've had both. Gen 1 PST glass isn't close to the quality NF SHV glass.

I am sure the glass is a lot nicer. Just wondering how they compare in low light. Will I see much of an upgrade?

Yes. The PST sucks in low light. The Gen2 PST is much better. I shot a hog well after sunset with my 5-25 the first time I took it out. The NF is slightly better than the Gen2 in low light.

You have the 5-20? Or the 4-14 f1?

I've had 3-9 and 4-14 SHVs. My 5-25 is a Gen 2 PST.

This is spot on.

I have owned or decent experience with all mentioned

How would these scopes compare to a Burris XTR II 4-20x50?

Burris has far more tactile turrets. Glass? I'd have to have then side by side. The 3-15X Burris I had for a year did very well in clarity and light gathering. Look at the weights of both scopes, I bet the SHV is a tad lighter.

wait for the XTR lll

I've got the 5-25 XTR II and the 5-20 SHV. Give me the Nightforce any day of the week and twice on Sunday. The biggest knock I have on the Burris is that, at least on mine, it loses some clarity out towards the edges. Enough to annoy me. But it does track every bit as good as all of my Nightforce scopes.

how do they compare at low light conditions?

Nightforce clearly has the advantage in low light, although if you're talking about deer where there's a defined starting and stopping time, either will do. If you're out really late like past the 30 minutes after sundown and want to shoot hogs, coyotes, etc, you'll be better off with the SHV. That's why its sitting on my primary hunting rifle while the Burris is on a target rifle.