Criterion #1--free. Both qualify. Why these two? SketchUp is well-known, and 123D Design is used by the guys on +Adafruit Industries 3D Hangouts (+Noe Ruiz an +Pedro Ruiz).
I bought a generator this Spring (Harbor Freight, 7000W) and decided to build a house for it. Since I needed plans, and since I am threatening to buy a 3D printer, I decide to use the opportunity to learn a design tool for 3D models. I started with SketchUp. There are some very good tutorials, but my complete ignorance really got in the way. I was able to create a model, and then a beeter one once I figured out that I messed up by leaving stuff around and inadvertently combining sections. Here's the second try:
This was good enough to build the house, but there was enough stuff that was wrong and/or didn't fit to make me want to try something else, so I went to 123D Design.
123D Design documentation is awful. There are a few tutorials, but they're short and basic. Using what I'd learned from SketchUp I was able to start. I needed the tutorials, because there are enough differences to require that. Once I got started, I went to one of +Noe Ruiz's "Layer By Layer" videos on the adafruit blog. I learned enough technique to be able to use the tool. Here's what I wound up with:
I made some more design decisions in the process, but I like this much better.
The moral of the story may be to keep working at it, but I think I'm going to stick with 123D Design. As I said, it's not a valid tool-to-tool comparison becuase I was learning through the whole thing, but I thing 123D has a better interface, even if it is undocumented. With +Noe Ruiz 's help, I'll master it (or at least get good enough).