I have both, toss up, the Viper is smaller, battery on top
Varoom battery on the bottom, pain in the ass, but it has more adjustment ( brightness) and a auto setting for light/dark, never really used it as I was shoothing that one in the winter in a indoor range and I would turn down for a fine dot for bulls eye. They are both 6 mm, I had a 3mm tasco type, great for indoor stuff, out side on steel, could not see the damn little dot. Would be fine on bulls eye paper.
I have the Viper on 1911 45 and the other on a 9mm Canik which I have sold , just wanted to try one, and then sell, friend played with it and loves the thing.