I honed my skills on 123d but have since moved to Autodesk Fusion 360, which I run on my Mac. Fusion 360 seems like a more advanced program than 123d, but to me, is more intuitive to use. the only problem i have with this program is in the 3D make prompt, 1) doesn't recognize my printer, CTC Duplicator, but does show Makerbot Replicator 2, which mine is a clone of, but I still cannot set it up as a dual extruder, and 2) the prints come out about 5-10% smaller than actual, so I have to scale them up to get the correct size.