I own both. A Nikon Buckmaster 3-9x40 and a Zeiss Conquest 3-9x40. You should compare the Zeiss to the Nikon Monarch as it is more fair. The Zeiss is hands down the winner in optical clarity. I like the adjustments on the Zeiss more as they feel more "solid". The Zeiss is certainly more scope but at twice the cost of a Buckmaster it's up to you if its worth it.
The one thing the Buckmaster has over the Zeiss is that the turret covers are made of plastic on the Zeiss and the buckmaster has very nice and soild metal ones. Seriously Zeiss, you're selling a $500 scope and you put plastic turret caps on, I have 40 dollar wal-mart scopes with metal caps. Nikon on the other hand has turret caps that other companies could stand to learn a thing or two from.
Oh and I dont like on the Zeiss how the soft rubber on the eyepiece has that inletted texture as it creats a poor seal for a lens cover and lets in water. Meaning when you flip open your cover and it has been raining hard there is water on the eyepiece. Perhaps I am nit picking. It is a great scope and I'm happy with it.