I have both applications, and use them in a commercial environment.  Phantast's comments on the (Renderosity) galleries are quite true -- Carrara Pro can do far better than the typical renderings posted in these galleries!  Vue is much easier to get productive with -- for pure landscape creation. Carrara has quite a steep learning curve (though I think easier than lightwave and a few others), whereas Vue is easy to get nice scenes generated in a few hours.

Mark's objective comments in the Carrara Forum were probably quite accurate.  Original content, landscape and modeling -- Carrara.
Pure landscape with imported models, and the need for lots of variety in vegetation distribution -- Vue 5 Infinte.

I find that Carrara seems to do better with indoor scenes or smaller outdoor scenes, whereas Vue shines in larger outdoor scenes.  Carraras renderer is close to that of high-end 3D packages. Also, SSS, translucency channel, displacement mapping, normal (bump) maps, etc.

Carraras volumetric clouds, and precise terrain layering and object distribution are excellent (but it looks like Vue 6 clouds will surpass Carraras?).  On the other hand, if you need to create a forest environment replete with hundreds of thousands of plants -- of differing species, nothing can beat Vue5 Infinite.

Seriously, if you can afford it, get both applications, as they actually do complement each other.