Following a hunt with South Nahanni where they did not even show me a sheep they would let me shoot due to lack of age, I did a ton of research over the next ~6 years on the NWT outfitters. The short list included MMO and ARRO, but I ultimately settled on Gana River and I could not have been more happy with my decision. Harold is not a flashy/salesy sort of a guy, but he impressed me with his honesty and candor. What initially sold me on him was that he gave the same honest answers to questions during the recession when hunts were tougher to sell as during the economic recovery (which I cannot say about all of his competition).
During my hunt I learned that he is the most honest and hard working outfitter I have ever hunted with. Case and point: while I was on my hunt, another hunter in camp blew out one of his boots and Harold loaned him his. When my guide sprained his ankle coming down to me and my sheep, Harold flew out wearing the other hunter's blown out boots and helped my pack out my sheep. To me, that exemplifies just how hard Harold is willing to work to ensure the success and enjoymment of his hunters.
He has a great camp too, with hydroelectric power being the differentiator over most others. With that comes the ability to freeze meat in camp, light 24/7, etc. The food and facilities were top notch as well.
The only significant differentiator as hit on above is the plane versus helicopter aspect, as Gana and I believe ARRO use supercubs rather than helicopters. The use of planes limits where they can put their hunters, and can result in a longer walk to game/back to the strip. My perspective was that sheep hunting isn't for the faint of heart to begin with, so a bit more walking was just part of the experience.
I'll be back up there in 2017.
If I can provide you any more color, feel free to PM me and I'll send you my cell #.