I've used both of them side-by-side on many occasions. We have about half of our house guns fitted with the Inline XR and the other half with the UltimateSlam. I've outlined the differences that we see below, but they are both great scopes. Also, the Inline XR is being updated with new looks and called the P3 Muzzleloader.
Glass Comparison: Overall, it is incredibly tough to judge which one has better glass. Both are extremely clear optics, offer good color, but I think the Leupold may be slightly better with contrast. The glass on these scopes will be comparable to your VX-2.
Eye Relief: The Nikon has about an extra inch of eye relief, but that has never been an issue. Because of the extended eye relief, the field of view ends up playing tricks on your eyes, because you can see so much in your peripheral vision outside of the scope.
Fit and Finish: Now that the P3 has aluminum turret caps, they are both very durable. We had a lot of issues with customers busting the plastic turret caps on the Inline XR. I personally like the look of the Leupold better, but that's completely personal preference.
The main difference is price, which the Nikon is $50 cheaper. If they were the same price, my choice would be the Leupold, but since there is a difference, the Nikon would probably be my choice.
Let us know what you end up choosing!
