I see a hierarchy here in these guns. The SP, the 640, and the 642 are not really comparable guns... they each serve a different niche.
I prefer to carry my SP 3"er in a belt holster. (3" barrel is my preference as this gun is NOT a pocket gun by any means. It carries as well as a 2.5" and offers a longer ejector rod, longer sighting radius, and just a smidgeon more velocity.) A bobbed hammer is helpful in that they are less catchy when pulling from under clothing, and this modification can easily be done with any SP. Single action shooting is not something you would do in a close encounter defense scenario... it's pull and start shooting as fast as you possibly can... period! So double action proficiency is essential with any of these guns... and that is something that doesn't come easily!
But there are many times when I don't want to bother with a holster and here is where the Smith RULES. Two choices:
640 for .357 power, but on the heavy side, if you can deal with that
642 for superb carry-ability and carry comfort: .38 spl +P power is the best choice in these lightweights.
Either (especially the 642) can be carried in a coat pocket. You can even hold the gun ready to fire it while it's in a coat pocket with no one around you being aware that you are armed and capable of instant action. The gun can be fired from within the coat pocket, if necessary, in a self-defense emergency.
I often drop my 642 in my front jeans pocket (where it basically disappears) or in the pocket of my motorcycle jacket while riding. Light, small, unobtrusive, no reason NOT to have this gun with you at ALL times.
Sealed, internal hammer is superb for keeping lint and gunk out of the gun, giving a super sleek profile while drawing, and making sure you get plenty of DA practice in when shooting the gun!
Note that I don't use the 642 for pleasure shooting... it's not made for that. But I do shoot it from time to time for familiarity. The all-steel 640 is a touch larger and a fair bit heavier which makes it not as great to carry, but more comfortable to shoot. I don't use my 640 or my SP-101 for pleasure shooting either. These guns are designed for carry convenience and not for pleasure shooting.
There is one exception here: a snubby which is a pleasure to shoot... and that is the Colt Detective Special, which is all-steel, the same size and weight of a S&W J-frame Model 60, but carries 6-rounds of .38 spl +P, is very comfortable to shoot, and is more accurate on top of that! Definitely one of the best; a legendary snubby.
So depending on what size and carry method you desire... you can pick your bullet launcher accordingly. They're all fine guns which will last as long as you are likely to need them. Don't make your choice based on which you think might be more "rugged".... they're all plenty rugged enough and most likely will outlast you. And if any of them should develop a problem, those are quickly and easily fixed. Pick the gun based on your desired size and need. And feel free to buy more than one gun if you can swing it! I think one really needs at least, well, 5 revolvers to cover one's based reasonably well!
Beyond the 642, 640, and SP-101... we graduate up into the Smith K-frames and the Ruger GP-100 frames... now we're talking CCW with shooting comfort and pleasure shooting capability. But that's another topic entirely.