I used Carrara 6 quite a bit over the last year, and loved it.  I'm just getting to know Poser bit by bit now.  I like doing portraits, closeups, etc, and use lots of Poser content.

Carrara still cannot handle Poser figures perfectly.  There were enough episodes of things that just wouldn't work in Carrara for me to finally invest in Poser 7, and not in Carrara 7.

I love the way Carrara renders.  Most materials I can get looking fantastic in Carrara... Except skin, especially closeups, render better in Poser.  I've just never been able to get the skin looking like I want it, and looking like I can get it in Poser.

Any sort of falloff or edge-blend is missing from Carrara materials, so if you like that aspect of Poser materials, you'll miss that.

I like Poser lights better, especially the options in the material room.  I like having separate specular and regular lights.  With that said, I seem to be able to light scenes better with Carrara, but I'm trying to learn Poser.

While most scenes render faster in Carrara, I found that clicking on GI, and/or AO in Carrara pushes it's speed slower that Poser.  I love the materials-based AO option in Poser.  I love having IBL's be light-based instead of scene-based as it is in Carrara. 

I like the results of AO and GI better in Carrara, but I'm not patient enough to wait for it all the time.

That's just my comparison.  Hope it helps.

Indi.