I had always wondered the same thing. Then I saw a great deal on a set of Hogue Rosewood Laminates to fit my Super Blackhawk. I didn't like the way that the rubber Monogrip fit the single-action frame but the wood Hogues look great. The biggest difference is that the wood Hogues are significantly thicker than the rubber ones. So you need a pretty good size hand for the wood grip because it is a real handful. I have since picked up another set of Goncalo wood grips for the Blackhawk frame. These have the finger grips, unlike my smooth Rosewood Laminates, and they are a thicker grip as well.
I also have or have had a number of Ruger double-action revolvers (Redhawk, Security Six, SP101). I almost always equip them with Hogue rubber Monogrips. I called Hogue to ask them if their double-action wood grips were similar to the single-actions as far as being thicker goes. The gal I talked to didn't feel comfortable in saying so outright but did acknowledge that there were some differences between the wood and rubber due to manufacturing differences.
Anyway, I'm very happy with the Hogue wood Monogrip on my single action and I am now looking for a good deal on a smooth wood Monogrip for my Security Six, and possibly my SP101 double-actions as well. They seem to fill my hand a little better. I'd sure like to hear from someone who's used the wooden Monogrip on a double-action though, to see if they are indeed thicker than the rubber ones for that frame style.
