I can give you a non-technical overview of the differences, having owned both.
In general, they are very much the same unit with the same tubes and similar electronics, but the 801s adds a few goodies like better black level retention and better G2 adjustment controls. The 801s is going to be newer and have more reliable components.
I liked my 800 OK, and I felt it gave a very good picture for the money, but the color never seemed to be "on" in every scene. When I tried to adjust the colors in one scene, they would be horrible in the next. I rented a color analyzer and played with that thing for a week, but it was an exercise in frustration.
My current Data 801s gives accurate color in all scenes. It seems to give better shadow detail and overall sharpness. This is subjective because I never ran the 800 and 801s side by side.
Bottom line if you have a choice between the 800 and 801s and the price is not a concern, go for the 801s.
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