Either one will serve you well. Not sure if you're looking at the FFP models on each?? The PST will have illumination...the SIII won't unless you go with their S-Tac line. Also, no zero stop for the Sightron.
Having said that.....I've been extremely happy with the SIII's I own. I just purchased a PST FFP a couple of months ago...so haven't used it much at all. I think the glass / coatings on the SIII's is extremely close to or minimally below the NightForce NXS line....when I've compared mine side by side. This makes them a strong value for the money. I've also been impressed with the accuracy and repeatability of the turrets.
As I said earlier....Either would serve you very well. I think optically the SIII might have a slight edge. But this might be totally subjective and others would say the opposite. It might just boil down to features and price?
Good Luck....you've got two great choices there.
Wayne