The difference between AutoCAD and inventor is that AutoCAD is just that, a CAD program. Whereas inventor is what is known as a solid modeler, since you aren't 'drawing' as you would in CAD, but rather, creating the 'object' in three dimensional space including it's physical properties, as you have stated. Also, solid modeler progrmas typically have a built in relational data base, such that when you change parts it auto updates the drawings or if you change a dimension on the drawing it auto updates the part.
However, as far as Solid Modeling goes, for the price, I would recommend Solid Edge or Solid Works, although Autodesk is working hard to catch up to them with Inventor. That is, of course, unless you get a deal on Inventor
As for 3D modeling, if I have accurate blue prints, I like to create a wire frame, similar to the pic below, in AutoCAD, then introduce it into 3DSMax and create the poly's using the wireframe as the reference. It's labor intensive, but when working from blueprints with actual cross sections and coordinates I like the control it offers.
