I just started working with Carrara (6Pro).  From what I can see and what I have read of other's opinions, Carrara is a much better renderer.  For one thing, it's much faster than Poser/Firefly.  It also has a huge number of options available that, in the hands of a skilled user, can turn out excellent results.

If you were equally skilled with both, Carrara would be the renderer of choice between the two, IMO.