Here's a quick comparison of V5 (left) and V6 (right) with no changes made to either.  Just double clicked the base figure of each, applied the Bree skin to each (there was a "V5 Bree All" in the material folder for Gen1 and "Bree All" in Gen2, not sure if there is any difference besides the naming), then applied the same pose to both and gave them both the same hair.  

My observations:

1. V6 is taller

2. V6's breasts seem more naturally affected by gravity

3. V6 has an uglier face

4. doesn't show here, but after experimenting I seem to be able to get more realistic and natural facial expressions for V6

5. the same preset pose applied to both, but the hands came out different.  I'm guessing that's one of the areas that had some major changes in the model.

Conclusion:  I don't see any huge leap from V5/Genesis where everyone needs to get this right now, but I do like the other V6 characters that came with the bundle, especially Norma who I'm using in another render right now. I'll post that when it's done.