as I use both, frequently
my reponse would be "it depends on what you want to do"..ie the details.
If you are creating an extremely large complex model (one model or just a few)..I'd say Bryce
Bryce can support far more detail then  carrara will ever open.

however, if its a terrain or some other model with extreme detail..trees, grass, and such..then its carrara (because of its replicators)..or explosions.

to clarify..carrara could never NEVER build this:
http://www.renderosity.com/mod/gallery/index.php?image_id=795134&member

but bryce will never do this:
http://www.renderosity.com/mod/gallery/index.php?image_id=1411091&member

both can be a pain..but both can do things that can inspire.
bryce is NOT as stable as it once was
nor is carrara as reliable as it once was either...its a bit behind the current technology but I'd anticipate it being updated soon.

bt