I own 3 Leupold and 2 Nikon scopes. The Leupolds feel a little more solidly built and I believe that they are. Leupold's service is outstanding in my experience. They reset the parrallex on a 1.5x5 within two weeks for free.
The Nikons probabley have the better glass. If you hold a Leupold Vari-X III and a Nikon side by side and look through them at a bright incandescent light bulb, you will probably see that the Nikon has less internal flare. This is a good quick test for any optic, both for original quality and for detecting optics that need internal cleaning, which is common in camera optics, which are not normally sealed as are rifle scopes. The Vari-X III glass is now "obsolete", which does no mean that it isn't just fine for our purposes. It is old single coated "bluish" glass. All the very best optics now use multi-coated "greenish" glass which passes a higher percentage of the light through to your eyes and less to image degrading flare. Leupold now has multi-coated glass in its new LPS series, but look at the limited choice and the cost of these!!!! The cost of the Nikon scope is even more astonishing if you realize that the glass they are giving you is probalby just as good as Leupold's LPS line. I don't own one of these, so this is just my guess.
I have never sent a Nikon rifle scope in for repair, but Nikon's camera service is slow and expensive. There will be no freebies and it is unlikely that one of their products will need service during the warrenty period.
The only reason these scopes from Nikon can be sold so cheaply is that they are being assembled in Thailand. Nikon has opened a Thai plant to cut the cost of selected optical products. It seems to be working out well for them. If these scopes were made entirely in Japan, they would probably cost more than the Leupold Vari-X III series.
These are all great scopes, not junk products. You won't go wrong with either.