you will find mixed personal views about both scopes. Here's mine:
Leupold 40x - an excellent scope that I struggled to range find with because the optic quality is so good that differentiating between 47 and 50m was difficult. Others have no such problem.
Nikko - I've used one and didn't like it at all (others will). I used to have a Mk2 and that was marvelous as it was user friendly - it was either blurred or in focus. I found the Mk3 to be similar to the Leup in that the distinction was not sufficient for me.
I recently sold my Leup 40 and the new owner is delighted with it and putting in some good scores.
I suggest that you also consider a Sightron. I have had one for a few months and rangefinding is easy as it performs like a Mk2 Nikko - it is blurred or it is clear.
Hope this helps