It's the subtle differences that make all the difference sometimes. Our beautiful model was presenting a challenge to me because whichever way i shot her she looked good. Now i'm not complaining about this issue but if you look at the image below with the black side of the flat you can see very clearly the effect both sides have. The white side here opens up the shadows in a very natural way. And because of the full size of the V-Flat you can open those shadows not only on her face but on her whole body.
V-Flat Camera Left - 60 inch Octobox Camera right
The black side on this image is actually more of my personal taste in this specific shoot. I like the drama with the shadows on her left. It defines her cheek bone better for me. The truth is it's really just a matter of taste. Plenty of times and probably more often then not I will go for the white side of the flat. From what I've seen the white side is probably used 70% of the time and the black 30%. But most photographers have the mindset of trying to add more light and figure out ways to add light as opposed to removing light. Subtracting light is what creates drama and drama is good. Obviously not always does one want drama but people need to start to explore more with light subtraction.