Carrara 6 doesn't come default with Transposer and will likely need it. For those that have C5 or Transposer and P5 or P6, C6Pro would be a nice suggestion, it might do better than Vue in some cases. For those that don't I would suggest Vue 6 (I personally was a bit disappointed in the capabilities of the plant editor and anything other than big wide open landscapes look just a tiny bit off). For anything but big wide open landscapes C6 looks a bit better (in my opnion), hair in C6 looks just great (although it has a few bugs).
So as usual the answer isn't clear cut, it depends on what you want. If all your shots are outside use V6. If most of your shots are inside with a few of the island (and you can get around the Transposer issues) use C6. For anything in between it's your choice. Currently V6 is more stable, C6 is very new and needs a few service releases.