When working on pieces (doodles or finished works) do you prefer using a pencil or pen? Or is there a divider between where one is more acceptable to use then the other?

I personally use pen for just about everything I do traditionally. I feel like the permanent marks help me learn from my mistakes and forces me to deal with what is already down on the page instead of manipulating what is only loosely there in pencil. It forces me to think about my space and use, which I think it a healthy exercise towards flexibility with work.

But that is just my experience with pen ^^ Your thoughts and preferences?