The S&W Model 10...
...in all of its iterations (including pre-Model 10 M&P's) was probably the most extensively produced handgun in the world. While it is not likely a collector item if it is not an unusual variant, I doubt that anyone who enjoys good revolvers will dispute my statement that any pre-lock S&W revolver that does not have a serious mechanical problem is a keeper.
As to four-inch barrel versus three-inch barrel on a carry gun, the biggest issue is whether you are short-waisted enough for the additional inch to slow your draw. With an inside-the-wasitband (IWB) holster, the extra inch should not make much difference in concealment. Assuming that the Model 10 has a normal S&W K-frame trigger stroke, choosing the Model 10 over the SP101 would likely give you a smoother trigger (a big asset when firing one-handed) and an extra round, at the expense of not being able to load Magnums. Over time, I have become convinced that most gunfights involving private citizens are likely to be at distances where the defender may be firing from some sort of compressed position where the added blast of a Magnum load may be a substantial liability. It's your gunfight.
