The Sightron SIII is a tank, I wouldn't worry about shooting it on a spring gun. I didn't do a side by side box test but both track accurately and are repeatable. The Sightron offers more versatility because you can power down if needed. There was some word out once that the Sightron SIII was a rebranded Nikko but that is not true. They are not even in the same class.
I am intrigued that you found the Sightron brighter. At what magnification did you check it compared to the Leupold? If my algebra is correct, they should have equal exit pupils with the Sightron set at ~47x.
I've heard some guys talking recently about the some issues with the Leupold sidewheels and recoil and that there was a need to go with something really lightweight in that regard but someone else will have to fill you in on that.
If I ever go back to shooting Open class, those two would be my scope choices. I like the Leupold because it's a simpler design with fewer lenses and therefore less to break. But it would need one of those super lightweight sidewheels from Eric Sanders. The Sightron is just good as is and certainly tough enough but you may or may not have some balance issues.
Man, that was a lot of jabbering that really didn't help. Good luck!!
Paul
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