Geoff, either of the Ruger models you've mentioned (the GP101 or the SP101) are fine guns, extremely solidly built, and fairly forgiving of beginner mistakes. The GP101 would be more fun to shoot, especially full-power .357 ammuntion, but the SP101 is quite manageable. You can also save some money and your hands by starting with .38 special ammunition in any .357 revolver -- the lesser recoil and flash will probably help you avoid developing bad habits.
I carry a close cousin to the SP101, the Smith & Wesson Model 60. I got it last spring, back during the early Obama Administration gun rush when you couldn't find an SP101 or most any Ruger anywhere. There are reasons that they were so rare, most of them being summed up with, "Good, tough, inexpensive." I don't regret getting the slightly more refined S&W; it's a wonderful gun. But I also don't think that you can go wrong with the Ruger.