I have a Big Nikko and an SIII.
Advantages of Sightron over Nikko:
It's a lot lighter in weight.
Sight picture is clearer and a lot lighter in dull conditions in woodland.
Range finding is more sensitive and snaps in just as well if not slightly better than the Nikko.
Never had any probs with temp variation (yet) on a par with the Nikko in this respect.
Turret is very positive (read slightly stiff) but with an oversized turret (as most peeps use for FT) this is not a problem at all due to the extra leverage.
PA wheel works in the opposite direction to the Nikko (just a matter of getting used to it)
Min focus on mine is 11.5 yards ( from what I have read this seems to vary a bit between scopes) but easily cured with a Leup close focus adapter on the front and I use one on the Nikko anyway.
The mil dots on the LRMD are a bit big but I have never really had a problem with this.
Hope that helps.
Daystate Mk4 iS ST (Huggett shroud & Mod .20) Lightstream 4.5-14x44 FFP
Daystate Grand Panther (Grand Prix stock, .177) Nikko Stirling Diamond 10-50x60
Daystate Panther (Gary Cane custom stock, .177) Sightron SIII 10-50x60