Well, it depends on the task at hand. For instance, I generally prefer ink, as I feel that my hand flows more smoothly that way. I have also become accustomed to ink due to the fact that the majority of writing tasks require its usage. I use pencil on the other hand when doing math
problems and the sort; after all, with math, that is a given.
Although... In general I like to use both of them for drawing, but that, too, depends on what I am drawing at the time. For example I typically use ink when drawing landscapes and abstract pictures. With abstract pictures the ink allows for me to somewhat mimic the technique of painters, while it allows for for the many lines and shapes of my "signature" labyrinthine works (they're not as great as they may seem).
For anthropological works - people, animals, and such - I typically use pencil. Aside from the sketching involved with said works, there is no real reason behind this; it is just what I have gotten used to, I guess. (or the real reason could very well correlate with the fact that I have lots of pencils lying around just about everywhere, and I mostly like to draw humans, animals, and humanoids.)
So... in a nutshell, I guess one could say that I don't have a particular preference in writing utensil. It all just depends on the time and situation.