You do not have to buy both unless you tryout Express first. Not sure about Express, but you can download Poser/Daz stuff directly into the Carrara6 Runtime Browser. And C6Pro comes with the Transposer plugin.

I would think that, for many, Poser or Daz Studio Apps are not really a necessity anymore.  Guess it depends a lot on how you work.

Check out the comparrison charts before deciding.

bwtr