There's a minor difference in Stainless steel version, it has about 2 extra milimeters of space in the head for the laser assembly. I prefer to use those to place that glow-in-the-dark O-ring to be held by the heatsink.
SS everyday of the week. Real quick question to Jerry....how is the black better exactly? I know you said heat radiation, but I am not exactly sure what you mean...
The black C3 is made of anodized aluminium. The material transfer the heat much better than the stainless steel version of the C3. It has nothing to with the color but with the material the host is made of.
I once made a 450nm Osram laser in a black C3. Smeerworst has it now.