Each Airweight design has a lot going for it. I like all three versions, but I bought a 642, for several reasons:
I am not worried about it not having a single action capability, because the thing is actually really accurate in double action - I don't need the single action capability. If I wanted one with single action, I would get a 637, because access to the single action hammer tip on the 638 is miniscule. My dad had a friend who got killed because the little hammer spur got away from her while she was lowering it. Yeah, the muzzle was also pointed at her. Yeah, that was dumb. It was over 30 years ago, I was a kid, and it made an impression on me that, logically or not, gives me a creepy feeling every time I look at a 638, so I have never really been interested in one.
I pocket carry my Jframe. The 642 and 638 won't snag the hammer on the draw, like the 637 can. I tried it. It can, whether or not yours does.
The 642 action is more protected from debris and dirt than the other two versions because more of it is covered up.
Due to the higher backstrap, it is possible to get the hand a bit higher on teh grip control recoil with the 642 a bit better than the 637 - again, I tried them side by side.
So: The single action hammer is immaterial to me, recoil control is better for me with the 642, I feel like the gun stays cleaner internally, and I get a cleaner draw when I want to slip it out of my pocket. 642 ftw, for me. It makes the most sense.
There's a reason they still make all three versions - different people like different things and have different priorities.