Not tried either myself but I've seen Rhino being used by a colleague and was impressed. I had heard somewhere that Rhino might be developed for the Mac but that was sometime ago.
I agree there are certain things that I find in other programs that deal with 3D better but VW gives me the ability to cover a wide range of problems including the more organic. I use machine design and work in the Film industry where the projects I come across can be obscure to say the least so I need a program that may not be the best at doing one type of design but gives me more creativity.
There are some great 3D tools in VW and once you get the hang of them the modelling can be quite diverse. i.e. the lofting tool is one.
Having said that, and I know it's been mentioned before, 3D contraints, collision of moving parts, 3D dimensions and assembly style contructions similar to SW or Inventor would go along way to improving the Machine Design module of VW. I sometimes feel, and maybe I'm wrong on this, that VW Architect gets the lions share of the attention. Then again for the price of this software compared to the other two I mentioned I don't really feel I can grumble.