In my opinion, the Sig Tango6 is an unappreciated good scope. It performs just fine. You will not find many Sig fans, but they keep showing and improving their optic line-up. What I mean is, I think some brands are just better than their general reputation in the market. Sig and Steiner are two that come to mind that are great optics, but people usually choose another brand.
I am not sure what I would compare it to. I have used it, and used the NX8 2.5-20, and I would say those are comparable. Better than a Vortex PST Gen2, and probably on par with Burris XTR3 and Vortex Razor Gen2, but not as good as a Gen3 or an ATACR.
One man's opinion. I do not have the details to give you of where the visual differences are. I can tell you as dealer, we sell mostly the LPVO magnification of Sig, and we stick to Tango6 in the higher magnification, as the Tango4 is not worth the uphill battle against other Asian made optics. The quality of the Tango 6 and resolution in higher magnifications seems very good.