I've had Vue 5 I for about a year now, and I still find it a mixed bag. When it first came out it was what I would define as an alpha build released to the public, for a pretty steep price. It is definitely more stable now, and if money is no concern then buy it.

However, I think Carrara's terrain modeler is better than Vu's, and the surface replicator in Carrara can do more accurate distribution of objects (can also covert instances to "real" objects).

If you need huge masses of vegetation with animated plants then Vue will be fine.

I personally think that Carrara's quite competitive as a scene renderer. Vue's algorithm for individual plant variation and animation are about the only thing I would consider as missing in Carrara. I do, however, think it is also easier to control sky colors in Vue, but then we have Carrara's excellent volumetric clouds, which beat out Vue's.

Download the Vue demo & try for yourself, but I personally think Carrara will do better with architectural scene renders. Big outdoor scenes with several hundred thousand plants belongs to Vue...at this point.