I wrote that the difference is very small. Here are the differences:
-Handling. The two binoculars have different shapes. You have to decide which one you like better.
-Field of view: Leica has 15 feet more at 1000 yards. It makes little difference, but it matters combined with other things.
-Curvature and distortions at the edge of the field of view. Nikon has zero distortion, Leica has a bit. However, Nikon has a bit more of the "looking through a straw" feeling, and the extra 15 feet of field of view in Leica, while slightly distorted, make for a more pleasant view.
-Focus speed and depth of field: The Nikons seem to have less depth of field and they focus faster, this may matter to birders because they may over-focus or may not be able to find birds as easily at close distances.
-Color aberations and color fringing: This is where Leica beats Nikon clearly. The image through Leica seems cleaner. Nikon will sometimes have a purple fringe aroud images.
-Ghost images and flare, viewing dark objects on bright backgrounds: they are both good, but Leica is a bit better.
-Contrast: Leica is better because it has less color fringing.
-Brightness. No difference.
-Focuser: Out of the box, Nikon has a very nice smooth focuser (it is greased), Leica seems stiff (there is no lubricant inside). In very cold weather, the Nikon gets stiffer (the lubricant may congeal), the Leica gets smoother. After 1 year of use, the Leica is very smooth and broken in regardless of temperature, the Nikon is stiff in freezing weather. The very smooth and unlubricated focuser on the Leica works better in freezing weather when using heavy gloves, it is wider and larger than the Nikon focuser, and is better exposed. I am talking about -20 Celsius/-4 Fahrenheit and bellow.
Edited by anweis