I'm still trying to answer your question myself...
If you're looking for a more traditional learn-the-craft type idea, I say go to the electric dept. When you're the one physically setting up and accomplishing the DP's lighting schemes, you'll learn a heck of a lot if you pay attention and always think about WHY (in a photographic sense) you are setting up what you are. Learn the tools of the trade and how to attack certain situations that you will find yourself in at a later time that may seem intimidating. You'll also be a lot more confident when you walk into a location on the scout and get a pretty accurate idea of what you are going to need, and the more you know about the lighting instuments, the more creative you can be.
Also, to be frank, I've found that DP's who have come from the electric dept. tend to get more respect from the crew, simply because they speak that language. As a crewmember, I have, sadly enough, worked with more than a few DP's who were pretty much clueless when it came to the best use of the lighting instruments and the gaffer basically became the DP.