Well, honestly I have Vue5 and I'm a bit fed up with e-on's policy. Instead of removing bugs, they worked on their strange content protection code, which messed up a lot of things. Vue is very expensive for European users, so I finally dropped it. I can do a lot of fantastic things with Carrara 5 Pro and I'm sure that Carrara 6 will catch up with Vue again. I don't have the money to support two midrange application at the same time and with Carrara I get a small modeler to modify imported things in Carrara. The render engine is also fast and I'm pleased with the results.

Vue6 landscape are not far ahead of Carrara, but I have to admit, that the presets are better. The atmophere effects of Vue 6 are awesome. This is really an outstanding feature.

You can get fantastic results in Carrara too, but you have to understand the shader system. In my opinion the best landscape renderer is Terragen, but this program lacks other features.

So Vue6 is certainly excellent in rendering landscapes and also supports Poser7
Carrara 5 Pro can also do fine landscapes and supports both Poser6 and 7(excluding dynamic hair and cloth). In Carrara 5 you can also model, morph, uv map and texture models.
Bryce 6 is an excellent application for a very low price. It does not have advanced features like Carrara or Vue, but if you look at beton's works here in the gallery, you can see what a real artist can do.

In the end it depends, what your goals are, what workflow you prefer and what amount of money you want to spend. Just use the trial versions, do see what fits best to your purpose.