I don’t think this is as trivial a question as it might seem. I like to use pen for many reasons. It is utilitarian, for one. It is permanent. Unless you spill coffee on your gradebook where you used fine tip flair. That’s when you’re glad you have everything saved on the computer.
With pen, I expect that whatever I write will stay put, so I use it for thoughts that I am confident in. Like signing greeting cards, recording students’ grades, and writing checks. I also use pen because it flows so smoothly. It’s soft and comfortable like my overstuffed chair. I use it for thoughts that are continuous and have the potential to be long and winding. Like commenting on students writing and taking notes in meetings. I used to use pen for writing too, but I’ve replaced the pen with the computer so that I can utilize features that I can no longer live without, like the right click to correct spelling and choosing font that fits my mood. I used to love the process of revising, scribbling out the permanent penned thoughts and squeezing in new ideas in the white spaces, only to want the first thoughts back on top.
I rarely use pencil unless I know ahead of time that I am going to erase. Not likely to but going to. Or if I’m left with no other option. Pencil gives you the opportunity for mistakes and to expect them before they even exist. I’m having trouble even thinking of the circumstances when I choose pencil first. Rarely. So I presume I don’t give myself much room for error. Or I just don’t expect to make mistakes very much. I think I’m just okay with words being permanent. And then I get back on the computer.