642 is the most practical for pocket carry because of its centennial style shrouded hammer.
The 638 is a very close second, only because I think the 642s shape is a little better for pocket carry. But i love the 638s look and design and it's ability to make a single action shot should the need or desire arise. If the need for single action is important to you on a pocket gun, this should be your first choice.
The 637 is out, an exposed hammer has no place on a gun that size.
$377 is a great price. You can find them a little cheaper on line, but once you pay shipping and the transfer fee it comes out to more then $377 + tax.