At work we use 3ds max for our visualizations so I am most comfortable with it. I usually do all my dimensioning in autocad and then import my splines into 3ds max to do the 3d build.
For the speaker adaptors I decided to build it all in autocad.
I want to get my hands on a copy of Inventor as it would probably be better for this sort of thing. Have used it a while ago with success.