I have both. When I carry a revolver, I tend to carry the 640 either IWB in a Galco Royal Guard or OWB in a Galco Concealable Belt holster. The 640 is very comfortable for IWB/OWB. For pocket carry, I go with the 642 in either a WRB or Uncle Mike's pocket holster. The 640 is just too heavy (for me) for pocket carry. It flops around and feels like....well, a big gun in my pocket.
The 640 has a slightly longer and larger barrel (full underlug), and comes with a tactical (pinky) grip. But you can put on a boot grip and it's as concealable as the 642. And if you buy a holster for a 640 the 642 will fit just fine.
Neither are good "range guns." That's not what they are designed for. Shooting .38's in the 640 is more comfortable, but .38's in the 642 are very manageable.
If I could only have one, I would probably go with the 642, because of the pocket carry option. If I did not pocket carry at all, then I would go with the 640.
I have a 642 and an sp101 which is steel. I carry both in my pocket. I'm considering buying a 640 because I find the little extra size and not the weight that I would like to be smaller.