My experience with my two nikkos, the mk2 is more estable with temperatures but not as clear as mk3. Mk3 has problem with high temperatures but ranges better than mk2.
I think sightron is very similar to nikkos but with a better glass. The things that i dont like is the mildot reticule, and the paralax wheel, is not designed for airgun use, with little space from 45 to 50m.
If the cost were similar maybe i choose the sightron.
The best reticule is the lrmoa iluminated..