Comment I'd recommend Rhino (Score 1) 218
I'm an architect and I've used Rhino for quite a while and have found it to have the most intuitive workflow between 2-d and 3-d, working between making curves or projections and using those to make solid geometry. It is also great at interface with both CNC mills and 3d printers--I've used it quite a bit myself to print architectural models and also know of quite a few other design offices that use it in a production setting. It also has quite a large community and a great scripting interface in grasshopper if you are into that.
If you are more into sculptural mesh modeling I'd recommend Maya or 3D-Studio Max but I'm not sure how it would interface with a 3d printer or CNC mill.
With regard to Sketchup or Blender or any other free software, I've never found their features or interface to be up to the level of commercial programs, and the cost for a Rhino seat relative to the productivity gains is negligible, and personally, while I'm sympathetic to open source, I'm reluctant to let software ideology preferences limit what I can do. If cost is a huge factor, though, and you can't amortize the software cost over a large number of projects, by all means look into those.