I've use Carrara 4 pro an 5 pro and like both. I love the speed of renders especially for animations. The ability now to pose poser figure in C5 is really nice. It has good modeling capabilities (though I am exploring Hexagon presently for modeling). I heartily recommend it. HDRI rendering available, Ambient Occlusion, lights are good, and as it has an interface similar to Poser/Bryce I found it a good choice for me. I do all my rendering in it.
Having said that, I have not yet tried Poser 6. However, I am guessing that the rendering speed is still an issue and though it has image based lighting, not certain if it is as efficient ac C5.
Have you downloaded the demo?