I have both 640 and 642 (and also a 342 Ti). I find a [I]big[I] difference in recoil between the 640 and 642. The 642 is downright obstreperous with .38+P. I wouldn't even think of shooting it with .357 Mag. But, the 642 is so much easier to carry. As we all know, the gun you have with you is better than the one back home in the safe. If this gun is supposed to be for carry (and not a "house" gun), then my preference would be 642, practice with 130 FMJ, carry +P.
I hate to sound like a broken record (broken CD? broken MP3?), but Pachmayr Compac grips are the best thing you can do to mitigate recoil on J-frames. Difference is night and day, getting a little rubber padding on the backstrap, between the back edge of the frame, and the unprotected bone in the thumb.