I would recommend the Kotonics over the model1sales upper.
From my experience, my first 6.8 was a model1sales upper. All the rounds I shot thru it with its 1 in 9.5 twist showed signs of high pressure on the primer. Trying to shoot the SSA Extreme resulted in several primers blowing out and even a broken extractor.
I then found this forum and Tim at Kotonics. I read about the modified chamber and 1 in 11 twist. Since I like the 6.8 round, I purchased an 18" upper. I have found this to handle the pressures better (allowing for higher velocity loads).
Also I found the accuracy of my Kotonics to be 1 moa compared to about 2 moa on the model1sales upper (which was shot on the same day, same ammo, and with the same lower. )
Hope this helps.
Steve T.