I see it more as two terms that are used by people with certain social personalities. People who are shy, introverted, and rather repressed use the term "dick" because it sounds less aggressive. Where as people who are extroverted, very expressive, and very confidant use "cock".

You can even see people use the word that seems to best suit the person you are talking too. Like my mom, she uses "dick" when she's talking to me (as I am her innocent daughter) or other people who are not as animated as she is. But once she gets around her wild ass friends, its "I like a big cock!"

Maybe that has something to do with it too.