Can't speak for others but, for me, what makes me choose Daz figures over others is the Morphs, Morph++ offers a huge arrange of possibilities.

Also, despite not having fantasy figures in my gallery, I render a lot of Orcs, Trolls, Goblins, werecreatures and the like, and V4/M4 offer a wide variety of morphs and products for that that I don't see in any other figures.

I'd switch to Genesis, I would even go through the process of converting V4 clothing to Genesis, but despite the DSON plugin, I'm not happy with how Genesis works inside Poser.

So, I'm staying with V4/M4 for now.