According to Cassino's definitions, I have tendencies of both. I'm a very academic Engineering major, I wear glasses, and I go through cyclic obsessions with particular video games or movies. :P
Subtract the glasses, and add the fact that I only intend to become an engineering major.
I feel more like a geek, but I have otaku-esque tendencies as well.
Nerds are persons obsessed with something, doesn't have to be technological or 'sciency', could even be flower arranging if you're that into it.
Geeks are the stereotypical bespectacled smart persons from American high school movies, well into computers and physics... usually similar enough to a nerd that the words are interchangeable with no ill consequence anyway.
I'm not either, but I thought I'd make an effort to explain the difference between the two demographs, assuming there even is one.
I think being an otaku is a geek trait too anyway. xD