Tom nailed it! I personally would say overall they are a wash. Just comes down to what options you want. About the only two complaints I have with Sightron, is they don't offer exposed turrets and the limited reticle choices. I also have a SII Big Sky and it's no slouch either. Both the SIII and Big Sky, fair nicely against higher priced scopes. They also are getting a reputation for having a Leupold like warranty!
If your into spinning turrets, the SIII offers 100 or more MOA of adjustment in all but one scope in that line. My Big Sky has 80 moa of adjustment and track very good. Zeiss tracks just as good from the ones I've used, but I don't think they have as much adjustment as the Sightrons. I could be wrong and maybe someone that knows for sure could post up, but I'm pretty sure it's not as much. The only other complaints I have with the Zeiss, is in the RpdZ reticles, the lines seem to close for my liking. I know people that think I'm crazy and are very happy with that reticle, but their just to close for me. The other is their customer service was so so in the past. That said, they could have corrected that by now, as it's been sometime since I've heard anything bad about Zeiss's cs.
Either way, your getting a very nice scope!