Depends on what I'm doing at the time.
Though, most of the time, I like using a pencil, because I tend to make a lot of mistakes...
and even if I DON'T make a mistake, I like to have the option to fix a potential mistake I might make along the way.
now, for drawing, I usually use pencil first, but then I go over it with pen. I don't like Pencil lines in my drawings, if I'm doing it seriously.
er- that is to say, when I'm using color. I usually go over it with pen, but I still dislike the pencil lines before hand.
However, if it's just a simple doodle, I will sometimes just leave it un-colored and in pencil.
for writing, I actually prefer neither. I prefer typing out my thoughts over writing them. But, if I have to, I would rather choose pencil, because paper does not have spell check, and I'm very questionable on my spelling sometimes. And, again, it's because I can correct mistakes with pencil. Can't do that with pen. With pen, the most you can do is scratch it out and make it look ugly, and I hate that.
And I know, there ARE erasable pens, but those aren't really pens >_>
to me, the meaning of "Pen" is "one that can not be erased" .... also, erasable pens always suck for me.
They never want to flow the ink out smoothly enough..... so yeah, I totally discredit them. They don't even matter.