I own both.  Carrara is an application that has a depth that Vue5I cannot match.  Terrain creation in C5Pro is superior, as is the precise application of instanced objects.  Vue surpasses C5Pro in its ability to generate hundreds of thousands of unique individuals of a particular plant and rock.

C5Pro is much more stable than V5I, but both suffer from the lack of a 64 bit OS and the subsequent  lack of sufficient memory address space.  I doubt that I will upgrade to V6I, unless there is good evidence that Vue cleaned up the V5I legacy code, and any new features will actually work -- before over 6 months after release.