I have the SIII 3.5-10 and had an STAC 4-16. I purchased the 4-16 after the 3.5-10 because I wanted the higher magnification at a similar size. (The SIIIs jump up a bit in size at higher mags)
After using both side by side I decided to keep the SIII and sell the STAC. The STAC glass (IMO) is pretty dang nice, close to the SIII, but the image is not quite as "real world" as the III. It's bright, crisp and clear, but the color rendition seemed more lifelike in the III - like looking out a clean window. If you never did a side by side, you'd never know.
For the money, the STAC is nice.
I also have a Bushnell 6500 elite 4-16 and the STAC gives it a good run for the price and out runs it in compactness.
Sightron claims (though I'm not sure I buy it) that the III series and the STAC share the same glass; the cost difference results from one being manufactured in Japan and the other the Philippines.
You'll be quite happy with the STAC.