As for the differences of the three models, you seem to have them sorted out. That is, the primary difference is the availability, or not, of single action shooting. The 638 with its hammer shroud allows both.
It all depends on what you want, I guess, and in this world of expanded civilian carry, S&W provides whatever the consumer wants. Part of what the consumer wants is shaped by perceptions of good taste and aesthetics--and I suspect that, subconsciously, many people think the shrouded j-frame just doesn't look good.
But since that kind of reasoning is not highly valued--e.g., the primary characteristic of a good revolver would rank appearance low on the list--the current more popular reason is that carry-ers might consider DAO, as there will be little time to cock and aim a firearm when one is in their mess. Personally, I carry a DAO version (M&P 340), but I even bought and carried a 360 to confirm my DAO decision.
I also rationalize my decision by noting that, once one is proficient in DAO shooting, there's no need to use SA shooting for self-defense scenarios: at distances under ten yards.
YMMV.
Jim H.
