I don't remember if it was last year or the year before but I had one of the small Dakine cable locks. It was nice because it was compact and fit in my pocket. However, one day I couldn't get it to unlock while my board was attached to a rack. I was able to pry the plastic body open on the rack itself and get my board free. Surprisingly, nobody even questioned me while I did it.
My point in bringing this up is this: If you're going to invest in a theft deterrent, I would avoid the plastic housing style locks. I had my board free in under a minute.
Going with a self-coiling style lock like what Rider161 posted above would be choice, even if it is slightly heavier to carry around.