The 22/45 is the same receiver and upper end as the normal Mark III but the grips and everything on the lower end is the same as a standard 1911 .45. They both shoot the same, the difference is that the 22/45 lets you get used to the feel of a .45 in your hand, by which I mean size of the grip, placement of slide release, etc. The big advantage of the 22/45 is it lets you throw pennies at the targets (.22) instead of the quarters required for a .45 bullet.
There isn't much price difference between the two types, but some of the regular Mark III's can be expensive if you go for the competition version, plated barrels, etc.
For plinking and target shooting, either would do great. Since you said this would be your first gun, I'd go with the regular Mark III, http://www.ruger.com/Firearms/FAPro...famid=55&variation=Target&bct=Yes&type=Pistol
This is the target version, so you have a longer barrel for better accuracy and adjustable rear sights. It's the same price as the standard and you'll shoot better with it, which will make you want to shoot more!
Have fun!
There isn't much price difference between the two types, but some of the regular Mark III's can be expensive if you go for the competition version, plated barrels, etc.
For plinking and target shooting, either would do great. Since you said this would be your first gun, I'd go with the regular Mark III, http://www.ruger.com/Firearms/FAPro...famid=55&variation=Target&bct=Yes&type=Pistol
This is the target version, so you have a longer barrel for better accuracy and adjustable rear sights. It's the same price as the standard and you'll shoot better with it, which will make you want to shoot more!
Have fun!