AutoCad is the industry standard for making models and drawings for engineers or draftspersons. 3D Studio is definatley more for presentation. 3d studio really doesnt offer much in the area of dimentioning or other things like that. If you really want to learn AutoCad, learn how to use it with almost no buttons, all command line (like Me!) its much faster, and you get alot more workspace
. 3d studio is beat however by things such as Lightwave 3D or Maya. these are High power 3d apps used mainly by game producers and movie makers. 3D studio however is used for most engineering and architectural purposes.
AutoCad is the app you want for designing your robot. Maybe inventor (ive never tried it, but ive not heard very good things.) AutoCad can do things in 3d, given you how (i diddnt know autocad had 3d for a whole semester!)
Use 3d studio for your animations. You may want to make your objects in autocad, it would be easier, then import them into 3d studio, but that doesnt always come up with good results.
Hope I helped
TENKAI