DaisyHead... glad I could be of some help.
For me, the 640 is THE Cat's meow, as long as the weight doesn't present any issues... it's just such a simple, solid, svelt, superb achievement in a small carry revolver that's a pleasure to shoot as well.
But if you need one to ride in a pocket, the 642 purrs as sweet as can be... I can even slip it into my front jeans pocket and pull it out readily... that's quite something, in my view.
For a midnight walk on the beach, a walk in the remote woods, through an urban downtown district... you can hold a 642 in your hand inside your coat pocket and look perfectly natural without giving anything away, and yet, if you had to, you could fire it in defense of your life right through your coat pocket, instantly... that capability does inspire nice confidence when you find yourself in potentially spooky circumstances.
Shrouded hammer designs can work for pocket carry, but their humpback shape adds a lot of bulk in the hammer area, making the gun harder to pull from a pocket and the shroud offers an entry point for lint, dirt, gremlins to enter and collect between the hammer and the frame - something that needs to be constantly checked and cleaned if necessary. There is a potential, however slight, for something getting in there and hanging up the gun. There are already enough things that can go wrong in a defense situation without adding yet another possibility for trouble... not a risk that appeals to me.
Bobbed hammer designs also have some capability of being fired from within the pocket as well, but there is a risk of the hammer catching on the inside of the pocket during it's retraction or during it's free fall - potentially hanging the gun up. Again, not an appealing risk to me.
So with respect to choosing a revolver design for pocket carry, I believe the centennial design (sealed internal hammer) is by far the best choice, though certainly not the only one.