It's a bit of apples and oranges.  Poser is primarily for character modeling and animation of people and animals and such.  It has some other tools, but they are secondary to the character modeling tools.  Carrara is basically for everything else, landscapes, skies, and other objects like starships, houses, toothbrushes etc.  What you want to do is do your character models in Poser then import them into Carrara.  This is a fairly painless process  And the results can be pretty stunning.  Take a look at my gallery for some examples of what you can do.  The renderer in Carrara is much more sophisticated and in my opinion, much better than firefly.  The indirect lighting options alone make it worth while.  Give it a try, but don't uninstall Poser, you will still need it for character models.

Quote - I've been using Poser since P4 (currently using P6).  I've played a little with Vue, 3DS Max, and am pretty good with Rhino.  Seeing the price on Carrera 6 got me thinking about giving it a try.  Is it worth it?  Is the rendering engine that much better than Firefly?  is the interface pretty user friendly?  Will it wash and darn my old socks for me?
Any input and advice would be greatly appreciated!