Ok just my opinion. If you plan to use it for personal protection either concealed or house gun I would go with the 640. Your probably not going to want to use the single action mode in a personal defense situation and if the gun doesn't have one, some lawyer can't say you did (causing the infamous HAIR TRIGGER) even if you didn't.
NRA Endowdment Life Member.....F&AM
Curio and Relics License Holder (FFL 03)
Gun confiscation will not happen in my lifetime.
"Of all tyrannies, a tyranny sincerely exercised for the good of its victims may be the most oppressive. It would be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." C.S. Lewis