Rhino is a NURBS surface modeler, it's not a solids modeler. You can make a sort of solid by attaching surfaces together, but it's hard to get things perfectly watertight, particularly if the surfaces have edges with problems.

C4D has its roots in modeling for animation, so it's good at making 2D renderings, but I doubt it's an optimal tool for creating solid models either. I'd say that if making 3D prints is important to you, that you should keep looking - there are lots of 3D modeling software alternatives out there these days.