That would be my personal opinion, that the skills you learn in the CAD side are very much transferrable. The CAM side of course is like chalk and cheese, but having got to grips with CAD at least that's one less thing to worry about.

It's worth noting that it's not identical - certainly you have more freedom when making a design for 3D printing as it is very simple to print things with very complex shapes that would be a nightmare to machine on a 3 or even 4 axis, sometimes just impossible even on a 5 axis. Though your results are generally going to be much better if you design with an idea of how it will be printed and design accordingly regarding overhangs and angles to minimise use of support material or optimise strength.

When designing for a router/mill, more thought needs to be given to how you are actually going to make it while you design. That's the main difference in my mind, but the actual process of getting what is in your brain out into 3D space is ultimately the same.