Terregen 2 will NOT import PZ3's.  I already asked.  So your Poser stuff would have to be imported the old Bryce way via OBJ & then apply textures separately(ICCKKK).  Vue6 has added clouds/terrain brushes/displacement/SSS/transluceny/eco-painting, although some of these items are not fully functional yet & won't be till final release IIRC.   So far from what I've seen the Vue renderer has vastly improved - new lighting models etc etc.  I've been playing with V6I for a couple weeks & love it....will love it even more when it becomes stable, as it stillis in pre-release.

I have Carrara & I think it does landscapes very well - it has most of the same features Vue has just presented in a different manner(displacement/SSS/trans/GI/GR/AO/Clouds), in addition to all the plugins you can accumulate help out a lot.

For vast landscapes I might suggest World Machine or Geo-Control( I prefer the latter) those top Vue & C combined IMHO. Those can be imported into either program inc. Terregen.