I love Genesis for when I want to create something not so human.  It has some amazing options for making different creatures and alien type stuff.  I've also had better luck getting realistic looking women with it, V4 seems to have some morphing issues whenever I try to add a bit of weight to her.

For everyday women I usually go with V4 because there are so many great, cheap morphs and skin for her.  

I like the updates on V6, hands and expressions stand out as a big improvement so far, but I wouldn't recommend for anyone to spend money on it if you're already happy with V4 or Genesis.