Hello,
I use Poser as well as Carrara. Be prepared that there are not that many free materials for Carrara
as for Poser, so either do them yourself or spend a lot.

Carrara offers replicators and good plants (look at the scenes from Howie Farkes at DAZ).
Carrara is very poor when it comes to displacements - you need to subdivide the mesh a lot,
which blows up memory and increases render time.
Since we have P8 / IDL, I find the lighting comparable between Poser and Carrara.

All in all, both are very nice programs, with different strengths and weaknesses.

Best regards,
   Michael