The 640 has nearly as much heft as the SP does and is built nearly as ruggedly. The difference in ruggedness between the Smiths and the Rugers (where it exists) is something that the vast majority of gun owners will never experience. Most people don't shoot anywhere near enough to experience a wear-related failure on any of these name brand revolvers. I wouldn't buy a Ruger simply because I thought it would "last longer"... they will all last a very long time indeed... almost certainly outlast you before needing any repairs of any kind. Smith & Wessons haven't earned their legendary status for nothing... they are superb revolvers. I seriously doubt you would possibly shoot enough heavy loads through any of these guns to cause a problem of any kind. And though problems do occasionally occur with ALL brands from time to time, they are easily repaired!
The SP would be a good choice (vs. a feather weight J-frame) if shooting .357 full house loads is important, because the larger size and added weight help moderate recoil. But the 640 isn't much different in this regard.
You can get a nice trigger on a Ruger (don't compare GP trigger feel to SP trigger feel because they are different and the GP is vastly superior to the SP for nice trigger feel). First thing is go one grade lighter on the hammer spring and the trigger return spring. Then polish the contact points. That will help a LOT in getting a nice trigger on a Ruger.... do that on a GP and you can achieve a really nice trigger pull indeed. The SP can get a lot better too, but just won't compare to the GP for "nice feel".
The bobbed hammer will present with less possibility of catching on clothing than a standard hammer, but won't be nearly as good for shooting from within a coat pocket as an internal hammer model will (640, 642). For this reason, I believe the 640 is a better choice than the SP, even with bobbed hammer. The 640 is slightly smaller and slightly lighter, but still a fairly heavy weight model for it's size and with the internal hammer, it's slick as a lick for pulling from clothing and can be held in the hand inside a coat pocket and fired from within the pocket if necessary - a feature which I give very high value to.
As you know, I've been following your quest for a small carry revolver and I still feel the perfect choice for you is the 640. They are very rugged, strong, powerful, concealable, the most snag free of any gun, and have the Smith & Wesson trigger, quality, reputation, and resale value! Not that I don't like the SP, because I do have and carry one of those from time to time and they're great little blasters. But I would give the top choice award to the 640.
