RV says they are different. I think they are nearly identical, but I know that others do not necessarily take into account all of the GC system. For example, if you use a light that is not set to white in the light color, then that color is anti gamma corrected by render GC, since it is non linear. If one were to only deal with shader GC, it would be easy to not remember that a light has a shader as well. The outcome will be different if that detail is ignored. So, if you are using shader GC, but you are being selective about its application (whether intentional or accidental) then a difference will become apparent. If you succeed in getting all the math right, then the outcome is nearly identical. Exceptions include objects with partial transparency, since there is no shader GC technique allowing us to deal with that identically. In such cases, render GC is more accurate. Other very subtle differences have to do with mip mapping and anti aliasing, but these are truly marginal differences, largely of interest only to purists. The big difference where shader GC just fails is when you use global illumination (IDL). With that, the shader becomes part of the lighting equation, and gamma corrected colors throw that off. With Poser Pro 2012 there is also subsurface scattering which is difficult to use right with shader GC. If realism is the goal, then you will want SSS and IDL, and render GC will give correct results more easily.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)