I’ve always preferred sketching UIs with an as-thick-as-I-can-find Sharpie over a thin ballpoint pen or finely sharpened pencil.
Ballpoints and fine tips just don’t fill the page like a Sharpie does. Fine tips invite you to draw while Sharpies invite you to just to get your concepts out into big bold shapes and lines. When you sketch with a thin tip you tend to draw at a higher resolution and worry a bit too much about making things look good. Sharpies encourage you to ignore details early on.
If you sketch, try a thick Sharpie next time. You may find you’re better able to focus on the concept and less on the drawing. That’s a good thing.