One could write a book on this but the short answer is: If you can get RayDreamStudio 5.5 for around $150.00 go for it. When Adobe puts there "Imprimature" on Carrara they will probably offer an "upgrade" to former RDS users and most likely fix the shortcommings and bugs with Carrara. I have both & still preferr RDS for buliding things and CAR for rendering. The shortcomings in CAR outweigh the new features as far as modeling is concerned (no nudge & lacks keyboard shortcuts and fewer options to control windows & workspace, no scene or modeling "wizards" and lack of shaders). CAR does render a little faster then RDS 5.5 however and the mesh-form modeler is more stable.