I agree with KagePen.
Pencil.
I'm naturally a very sketchy drawer. I draw something, then draw something again over it, then again. Imagine the evolution of something, how it slowly refines itself until it ends on a note nothing short of magnificence. Though, my drawings are nothing magnificent (yet), you get the point.
Without the ability to be erased and redone, my drawings would be nothing more than a fantastic blob of messy lines. So, I prefer a pencil.
Don't get me wrong, pens are wonderfully clean. They are great for line-work and definitely make a permanent impression. You won't ever have pen "smudge" over time like pencil does when transported, shifted around, or rubbed.
Just my two cents...