My first post here was on these two firearms. I just got the 642 about a month ago. It is an excellent firearm and the weight is nothing. I've carried Glocks for years now, and they're light as well but nothing compared to the 642 at 15oz.
Having said that, I plan on picking up a 640 this Saturday. It is NIB at $460 with both styles of grips. I'm very happy about this as well. My first off-duty was a .357 and now 20 years later I've come full circle.
In regards to ammo, as I mentioned in my other thread, I've heard really good things on the Speer GD SB loads particularly in .38+P. I believe both the .38 and .357 rounds are 135 grain which should allow either to hit right at or very close to POA as these snubbies are usually best with the heavier bullets i.e. 158 grain.
I understand and appreciate the comments on carrying the lighter high capacity semi-autos. The majority of my career was with a revolver i.e. a S&W 64 .38 Special. Great gun. Then we went to the Beretta 9mm and now we use the Glock 21 .45ACP. All great firearms. My off-duty carry somewhat mirrored this as well in that I started off with revolvers and then went with semi-autos. In my case I really had to for many years as I taught in the Academies and most local agencies either used Beretta or Glock.
These days I don't teach firearms much anymore, at least in an Academy or in-service setting. And I just like the revolver for its simplicity. I've had to draw my sidearm four times now in my 20 years. I've learned that under duress, simple is simply best. That is why I've gone back to revolvers for my personal off-duty carry.
If one is to carry a semi-auto though, no problem. But please do the following for your own benefit;
Make sure you can clear any type of malfunction that may occur such as a stove-pipe, FTF (failure to feed), FTE (failure to eject) or double feed. Especially under duress, which you'll be under in a real world altercation. Make sure you can do so fluently and without hesitiation.
Make sure you know how to fire with either hand, as your primary hand may be injured or busy with some other function. Some folks tend to limp-wrist with the 'reactionary' hand if they've never used it to fire the weapon.
Make sure you know how to cycle the slide manually, with either hand....or without using your hands at all!
Make sure you know how to load with either hand fluently.
Just a few tid-bits to consider.
My advice on the 642 vs. 640...both
If you can afford them.