The 638 has a shrouded hammer that is accessible for single action. For pocket carry a lot of lint will get in the shroud (I know, I had an alloy version).
The 640 has an enclosed hammer. It can only be fired double action. There is no access to the hammer.
For personal defense I can think of no situation where single action is appropriate. Additionally the spur to lower the hammer on the 638 is very small and makes that operation extra risky.
Also look at the 640 Pro. It comes with a real rear sight, an XS front night sight and moon clips for rapid reloading. I think this is a better choice (I'm going to buy mine this afternoon).