My teacher back in the days said. If you use a pen, a pen will force the direction of drawing more than a pencil does. With a pencil you can draw way more detailed and softer than a pen. Even a little touch of the pencil will draw something. With a pen you have to push a bit to let it draw so it will force you to draw in certain directions. And now it depends on what you wanna try to achive...
My teacher back in the days said. If you use a pen, a pen will force the direction of drawing more than a pencil does. With a pencil you can draw way more detailed and softer than a pen. Even a little touch of the pencil will draw something. With a pen you have to push a bit to let it draw so it will force you to draw in certain directions. And now it depends on what you wanna try to achive...