As others have said, it depends on how Bob wants to carry. I have both a 640 and a 60. I carry the 640 most of the time because it fits into my pocket and the extra weight doesn't bother me.

My 60 is the Pro Series model with a 3" barrel, that when I carry it needs to be in a holster strong side. The 640 is just easier for me. BMMV (Bob's mileage may vary)!