Here is a post with multiple online resources:
http://archicad-talk.graphisoft.com/viewtopic.php?t=36111
I have found that ArchiCAD is relatively easy to learn on your own and there are tutorials and tips galore. That being said, it will take you a long time to go the self-learning route and you might develop some bad habits.
The three day course will cover a lot of information in a short time and probably get you reasonably productive quickly, haven given an overview of the key parts of the product. I am guessing it is offered by a re-seller or consultant. This is a plus because they most likely are closer to the industry and real practices out there using the product and you will most likely learn the latest version of the product
The eight week course is going to be the most in-depth, if the instructor is any good. And that is probably the key to deciding. Get an interview with the instructor to see what version they are using, what types of projects they are going to design in class and if he/she has used ArchiCAD extensively in the real world. If the instructor is a good one this makes all the difference.
