I have used nikons for a few years but three years ago I made the switch to vortex.
Both are great scopes but for me the vortex lines are better.
I started with the diamondback but have since upgraded to the diamondback hp and as much as I liked the diamondback the hp - models are that much better.
As for as warrenty I have used it three times due to scopes getting broke from falls from tree stand and getting ran over by a four wheeler by a idiot driver and vortex never asked how they just said ship it in and we will repair are replace and all three times they sent me a new scope.
You really can't go wrong with either company's scope I just like vortex better for what I do with my rifle.
Before you buy you should sit down and really decide what you want the scope to do, hunting only, target and hunting, mostly target work with a few deer hunts.
The most over looked detail about a scope is how long it is and how much it weight's.
Hope this helps you sir.