My opinion of these similar revolvers? In two words: Most favorable!
As I understand it, the DA Sixes were costing Ruger too much to manufacture, so the simplified GP100 was developed. The cost of this simplicity, in seems, is more bulk. Because the original-pattern GP100 factory grip is such a superb fit, in my hands, and mitigates Magnum recoil, I reckon that the newer revolver is “worth it,” in my case. Others’ experiences and optinions may vary.
I have two Speed Sixes, 4” and 2.75”, acquired in that order, about a number of years apart. No sane amount of money could persuade me to part with either one. The 4” Speed Six wears a Pachmayr Compact Professional grip, and the shorter one has the Ruger factory wood.
My several GP100 revolvers have barrel lengths from 3” to 6”. My first is visible in my avatar; an early Nineties with a full lug and adjustable sights. I used to to fire a necessary defensive shot, in 1993. (Necessary, because somebody was about to be seriously injured.) Sorry about the seemingly unremarkable avatar photo, but I wanted to show how I hold quite very high on a revolver’s grip, to mitigate recoil and muzzle flip, on another forum. When I decided to experiment with adding an avatar image, here on THR, I saw this image, in my iPad’s saved favorites, and thought, well, this is my most-favored firearm, in the world, so, why not?
The Speed Six is a bit more concealable, and the GP100 is kinder to my hand, if I fire Magnums. These are both wonderful weapons. Life is good.