The S&W website mentions three distinct hammer styles: the "Chiefs Special" with exposed hammer, the "Centennial" frame with fully enclosed hammer and the "Bodyguard" frame (.38 S&W Special +P only) shrouded hammer.
Do you have preference? If so, which one and why? Or is the j-frame you mention essentially the "Bodyguard" frame?
The 642 is a Centennial frame with fully enclosed hammer. I like the fully enclosed hammer style best for pocket carry because of less concerns with snagging on clothing or pinching. An Uncle Mike's pocket holster works great with the 642.