[ChrisH]

If you have to pick, I'd say learn Inventor. While Inventor does not have a whole lot of seats out there in comparison to AutoCad, the way it works is the direction most major CAD systems are going. I find it very easy to switch back and forth between Inventor (robotics) and Catia5 (work). (I don't want to talk about Catia4, the memories are still too painful) Even Unigraphics is slowly getting more parametric and sketch based. This is not to say that there is no place for AutoCad. I still fire up my version of R14 when I need to answer questions using just wireframe and surface stuff. But I find I'm doing it less and less.

The key is to learn a reputable parametric solid modeler. Once you've figured out how to work in one, the rest are easier.