I don't know about the S&W, but the Ruger MKII target gun should have 4 screw holes in the top of the receiver for a scope mount. I don't know what kind of rings it uses at that point, but you should be able to mount any type of scope you want, including a red dot.
If you have an older Ruger MK2, Burris sells a mount that requires you to disassemble the rear target sight on the gun. Then you slide a collar around the barrel, and finally screw this collar to a scope base that also screws into the rear elevation screw in the rear sight. This mount doesn't work UNLESS you have the target sighted gun.
Red dots are nice because you don't have to worry about parallax like you do with a regular scope. Just line up the dot and squeeze the trigger.
I also like the 2x scopes on this kind of setup. It has almost at much field of view as a red dot, is real easy to pick up the crosshairs and has a very small parallax error to be concerned with. The only bad thing about a 2x scope, or any other scope is that they tend to be blurry at most reasonable pistol shooting distances.
One thing you might want to look into getting one of Mr. Marvel's triggers on the gun. They are nice because they are completely adjustable for trigger length, twist, slack and let-off.
Also, the Ruger is a GREAT squirrel gun!!!