I used to slightly prefer the 6x42, because Leupold started multi-coating its lenses in the early 1990's, while the 6x36's lenses remained multi-coated. With the multicoating and a larger objective lens, the 6x42 was noticeably brighter in dim light.
But since the fixed M8 Leupolds became FX's, the 6x36's lenses are multicoated too, and while there's still a difference it isn't nearly as much. Since the 6x36 is noticeably lighter and slimmer, I tend to use them more often than 6x42's anymore.
