Your 642 weighs about 15 ounces while the 640 weighs about 24 ounces. The 640 will have less felt recoil than the 642 with the same ammunition.

As to 357 Magnum, if you have to ask about recoil of 38s in an alloy frame revolver, you will not like the recoil of a magnum in the 640.



As to ease of carry that cant be answered without knowing how you carry.

If you pocket carry, yes the 640 will be the better choice. If you are carrying in a standard belt holster there will be no perceivable difference.

Only you can decide if it is worth adding.