I think NetBSD is a good choice, because they are the most friendly to people experimenting and trying things out, which is crucial for learning. The OpenBSD people are very dogmatic about having everything in the system the way it is and not any other way. That is not to say, you can't try out things in OpenBSD, but you will fall on deaf ears, once you go a bit "off-road", in my personal experience. NetBSD docs are goods too.
While I have to admit, that I'm not a kernel developer and hence don't know how lightweight NetBSD is compared to other BSD derivatives on the inside, I can say that my machine idles at around 150MB with Xorg, a window manager and some daemons running. Compared to other BSDs NetBSD feels snappier to me too.
