There was an article sometime ago in Outdoor Life magazine. In fact, I recently saw the article on a website.
In any event, the article compared various scopes including the Elite 4200, Zeiss, Grand slam, BSA, Swarovski, Cabellas, Kahles, and other scopes.
The Elite 4200 and Zeiss had the best resolution at SOA 3.3 (whatever that means). BSA was junk, Cabellas wasn't all that great, Kahles was somewhat of a disappointment, especially given that it had 50mm objectives and was quite expensive.
Anyhow, the "editor's choice" was the Grand Slam. And I think it was the same one you are looking at. Part of their decision, in part, was based on price and mechanical functioning. I have heard that the tracking on the Grand Slam is excellent.
I have never even held a Grand Slam in my hands, but I do know that Jon is quite fond of them (as is, apparently the editor at Outdoor Life). I would be tempted to say that perhaps Weaver, or other manufacturers, have inside deals with magazines and/or their editors to make a decision. However, if Jon also likes the Weavers a lot, then I am more inclined to believe the accuracy of the Outdoor Life article.
Still, I would recommend the Nikon Monarch as I have owned several and NEVER had any problems. (In fact, I recently purchased, and received, a Nikon Monarch 3x-9x-40mm, and Elite 4200 4x-16x-50mm, from Jon just this week). Of course the Elite 4200 are superb as well. The Vari-X IIIs are great, but, in my opinion, overpriced, and really not as great as the Elite 4200s or Monarchs in terms of optical resolution and clarity (I also own a couple of Vari-X IIIs).
If I find the article, then I will try and post it on this board.
Zachary