I used to always just use a pencil and paper, but once I became proficient in Autocad years ago, I use it for most 2d parts or cnc plasma projects. Simply because I can do it faster and more accurately. Plus the ability to print out different views to scale helps with some projects especially if you wand to do simple templates to use with a punch for drill hole placement etc. For 3d parts for the CNC mill or 3d Printer, Inventor is the ticket. With it's ability to do the CAM and output your machines g code file (or and STL for the 3D printer) it's hard to beat.