It basically boils down to this:
Hexagon is a fast stand-alone modeler.  It does not have any animation or rendering capabilities.
Carrara is a full 3D suite, with modeling, animation, terrains, shading (and more) plus rendering.

Of course, this is over simpliefied, but it should give the basic idea.

Best,
Bob Stockwell
Director, Sales and Marketing
Eovia