I have a couple of 5 ½” Redhawks, in .44 mag and .45 Colt and a Super Redhawk with a 6” barrel. The grips are different. That is a personal thing. The mechanisms are different, as has been said. What it boils down to, for me: the Redhawk takes some work to get a pretty good single action pull, the Super Redhawk can get a very good single action trigger pull very easily, but the double action pull is not as good as the Redhawk.
Like most things in life, it is a matter of personal preference.