S&W uses MIM, Ruger uses investment casting. The material, machining and heat treating probably make more difference than how the formed, but unfinished part is made. I like both the cast parts Ruger uses and the MIM that S&W uses. The cast parts are through hardened, smooth up nicely to provide a very nice trigger pull and typically don’t require much fitting to replace things like the hand. MIM also seem to me to be harder than the older parts that were forged. They are also, in my experience, just a little rougher, and a little harder to smooth up than the forged and case hardened parts, but when they are smoothed up, they provide a better, longer lasting trigger action than the forged parts. Most of my newer guns have the MIM parts and I work all of them and when I am done, they have very nice triggers.
I have heard that Ruger has changed their processes for making cylinders, but I don’t know any details. I have also read that when S&W changed to CNC production techniques for cylinders, they significantly improved the quality and consistency of the cylinders. I think good cylinders have a lot more effect on revolver accuracy than MIM or investment cast internal parts.
I am very pleased with all my Ruger and S&W revos. After breaking in the barrels, all have been very accurate. I also measure the chamber throats on all of them with pin gages to see the size and chamber to chamber consistency. All the newer stuff is very good, while the older Ruger single actions in .45 Colt tended to be tight and benefitted from reaming the throats.