I was on the fence between those two scopes about a month ago. Some of the reviews I read gave a slight optical advantage to the SIII, most said that they couldn't discern a noticeable difference, and a couple said that they would give the edge to the PST.
The advantages of the SIII are:
- more elevation travel - 100MOA vs. 65-70MOA of the PST
- possibly a slight edge in optical clarity - debatable
- possibly a slight edge in mechanical repeatability of the adjustment turrets - debatable
- 15MOA per revolution, compared to 12MOA with the PST
The PST has these advantages:
- available in FFP, which was important to me, since I use the reticle for wind holds
- zero stop is a nice bonus
- illuminated reticle is also a nice bonus, albeit unnecessary for the shooting I do
- slight advantage in warranty
- better reticle, IMO
- better eye relief
- not sure about Sightron's coatings, but Vortex has a scratch-resistant coating on the PST, which is nice
With a 20-30MOA rail, the 65MOA of elevation that the 6-24x PST has (my scope has 70MOA, FWIW) is plenty. For my purposes, the PST FFP was the better choice.