Sheets for me, dot-matrix printer paper just lately.
You still have dot-matrix paper?!
I recently came across an old school assignment from a storage box in the basement, and this sort of school paper was (at least in my case) of very bad quality. If I ever put an ink on it, there would be awful feathering. Good though to see you're getting some mileage out of that source.
Regarding the original question, I stick to my watercolor paper sketchbooks because loose sheets always end up scattered all over the place and I dislike that. I also like the chronological aspect of the sketchbook, I can see my progression (or lack thereof
) Besides, when working with watercolor ink, there aren't too many cheap alternatives.