I've owned both Nikon and Leupold glass over the years and never had a problem with either. Although, I believe the Leupold glass has a slightly sharper image. Admittedly, this point is arguable.
With that being said, I do own a pair of 10 x 42 Leupold Olympic binoculars. They are very rugged and hold up through a lot of abuse (or shall I say hard use.) The only thing I might change about them is the eye piece. I believe the design could use some improvement in that aspect. The eyeglass vs. non eyeglass wearer portion gets worn and gets to the point where it moves in and out too easily.
If I had it to do over again, I would've bought the Nikon's, but that is only provided they had a more robust eye piece than the Leupold's. The best way to compare is by visiting a sporting goods store and checking both of them out.
Every year my Pro-Staff and I head to the woods to hunt the old wise Tom. We travel thousands of miles covering numerous states hunting these old guys. Lots of times we end up hunting public lands or private property that have been hunted already. That’s the last thing you want to hear when you just traveled 6 hours that someone else has already hunted the property. We had to come up with a game plan and one real fast.
The first thing we did was ask the farmer were they usually see the birds in...