That is the question.
They both serve different purposes, and leave a different impression on the paper. I prefer pen when I am editing or marking up a document, and prefer a pencil when I am jotting down a list, writing down ideas, or simply drawing.
A pen leaves a permanent mark on paper, whereas a pencil allows for soft and faint lines that can be erased. The latter can even lend itself to smudging and light shading for drawings and sketches; although it’s possible to do so with a pen, it cannot be done typically with the same ease.
But if an artist masters the pen, they can create stunning works using different colored pens, pen-tips, boldnesses, and use cross hatching to create vivid and crisp and detailed images. On the flip side, an artist with a pencil can use a variety of graphite to achieve stark contrasts and a wide range of tones, and make as many mistakes as they need to, since they can erase them or simply draw over the line before the final draft.
Using a pencil is also more preferable for doing math work, since it would be a waste to write down a ton of steps using a pen, only to realize that one mistake negated it all, and then be stuck with all these symbols and numbers that are un-erasebale. Setting everything aside, there are countless examples in which one or the other would be better (or preferable) in certain circumstances. But whichever one prefers, it depends on what it’s for, such as editing, drawing, doing math, etc., and knowing how to leverage their strengths.
Fallton Havenstonne 