A 642 weighs 15 ounces. A 649 weighs a bit more than 22 ounces. In your pocket or on your belt you will never notice the extra seven ounces. But the two guns are night and day on the range using similar ammunition. You can and will be more accurate with less "work" with the all steel model.

I retired my 642 and replaced it with my 649. There are days when I carry a 2" K frame, which makes a big difference but, if one sticks to J frames, the Model 649 is my pick for the way to go. YMMV as always.