The stuff from Circle K is usually 3-4 brands (Revolution, G2, Ten Factory, Alloy USA) and they are made in South Korea. Yukon I believe used to use Circle K but in the last few years started their own thing in China. I believe the stuff from China for Yukon is known to not be as good of quality, which would explain Sierra's findings. The Yukon stuff is also extremely expensive in most cases.
As for other brands like anything from Randy's, Rough Country (from Randy's), Motive, and a few others, I'm not sure where they come from. Motive has been pretty much out of stock just like Revolution so they may be from S. Korea also, although I know their 4.88 ratio uses different tooth counts than Revolution so I doubt they are from the same place. End of the day it doesn't really matter where you get from if they are all out of stock, mainly about just finding them at that point. There are some differences amongst the brands though, the only brands that are the same are the ones that come from the same places, like the 3-4 confirmed brands that all come from Circle K, and as already mentioned, are currently out of stock.
I like Revolution because I know they are from Circle K and always will be. If circle K is out of stock, then Revolution is backordered no matter what. I would trust Motive just as well but they are out of stock also. I would probably go to Randy's or RC after Revolution/Circle K if I needed gears while the Circle K stuff was out of stock, assuming Randy's/RC are available in stock.