I've owned the SS 1-6 as well as the Razor and Mk6 (but not the USO). The SS 1-6 is a very good scope overall, but it has relatively narrow field of view at both 1x and 6x, which is a big drawback for a low magnification scope meant for a carbine. The Razor and Mk6 have wide fields of view in comparison, although some European scopes are wider still (Steiner and IOR for starters, probably others). On the plus side the SS is not as heavy as the Razor (which is a brick). Its glass quality is good, but the eyebox is small - very sensitive to eye position. Good resolution with your eye perfectly centered became blurry with your eye only slightly off-center. I didn't warm to the donut reticle, largely because it meets the outer edge of the FOV at around 5x, so you can't really use 5x, only 6x or less than 4x because of the area being blocked by the donut. The T-reticle they offered in the 1-4x would have been superior, IMHO, but was never offered in the 1-6x. The turrets are fine but I don't see lots of dialing with a 1-6x scope.
I found the Razor to be excessively heavy and I found its glass clarity not up to my expectations for a $1400 scope. It does have wide FOV, a much larger eyebox, and a simpler SFP reticle without the SS's issue.
I had the Mk6 with the CMR-W 7.62 and thought it was an excellent scope in most ways. The only reason I didn't keep it was misgivings about the ruggedness and durability of the turrets and the ocular focus adjustment. They kept the weight down but it did not feel as rugged as anything else in that price range, including the Razor and SS (both cost a lot less, actually). This is purely FEEL and I never had actual problems, I just didn't have confidence in it. It had awesome glass and an awesome FFP reticle, along with great illumination.
I ended up keeping, and getting a second one, of the Burris XTRII 1-5x scope. It has illumination comparable to the Mk6 and better than the Razor or SS, a SFP reticle that is small and thin if anything, and quality that felt good, especially relative to its much lower price ($800 or less). Also pretty good FOV. The XTRII's glass is excellent in the center, but has noticeable distortion at the very edges. Still better than being completely blocked like the SS's donut at 5x.