After some more thought, I've decided that an SP-101 is a good all-around gun. This assumes that (1) you're not going to hunt with this gun, (2) it is your only handgun, and (3) you are going to carry this gun.
The SP-101, from all that I've read, is the ONLY snubby that can shoot .357 Magnum cartridges fairly comfortably and all of the time. It is easy to shoot, easy to clean, and relatively easy to carry. The jump from .38 +P to .357 magnum is huge. You are effectively jumping over the 9mm, 45 ACP, and other cartridges when you load your SP with full-house magnums, and the SP was designed to shoot them regularly.
While you give up capacity to the 9mm and, to a lesser extent, the 45 ACP, I can't imagine needing more than one more-or-less COM shot from a .357 mag. to end an encounter once and for all. It's a simple design, and in the DAO configuration probably the safest type of handgun you can own. When you get tired of firing .357s, you can switch to 38s and practice weak-hand and one-hand shooting. You get the full range of 38 to 357 cartridges in a compact package that is relatively easy to practice with. A S&W 642 is no doubt easier to carry, but it will not shoot Magnums, which are roughly two and one-half times more powerful. The SP is a different beast. The fact that you can carry this beast fairly easily is a testament to its greatness.
If you want a .38, go for the lighter, two-finger models. If you want a hand-cannon, get the SP, shoot it regularly, and deal with the extra size and weight. I think the 5-round capacity in a 125-grain .357 magnum is more than enough.
~Ichiro