Depends on what you need it for in the end. If its straight 2D or 3D CAD, then you don't necessarily need a top of the line CPU. A mid range quad core or even dual core is good for almost all CAD programs. However, if you plan to do any engineering simulation or rendering, you are better off with more cores as those types of programs will utilize more than CAD. Also, if temperature is an issue you can always add a water-cooler to the CPU. Having a good GPU will help too as it will even out texture, rotations, and models when working on complex builds. RAM will also be a factor for CAD, perhaps the most important factor. You'll want at least 8 GB. Best bet is to find out what the software you're using recommends as minimum requirements.
Bottom line, brand doesn't matter as much what your bottom line requirement is. If you're on a budget AMD is probably the way to go.