To the people who struggle with sketching digitally: are you using a screen tablet or a no-screen tablet? I couldn't sketch properly on a tablet until I got a screen tablet.
Once you have one, use a pen nib that creates lines that ease your mind for sketching. For me, I tend to relax more both when inking and sketching if I use bright clear colors and a maru inkpen nib. I think it's because it feels like I'm using red/blue ballpoint pens and just making little pictures in my sketchbook for my own use. Removes the whole stress of "this has to be representable!!!!!" (you know, that stress that ironically enough often renders the work less representable). What works for you will probably be different due to individual associations.
Use low/no stabilization for sketches, use stabilization for inking if you are aiming for a clean look.
Sketching digitally can be very beneficial, especially if you plan on publishing a finished piece because you can quickly fix any anatomy problems. As for doodles, it's once again nice to be able to do it digitally just in case you end up liking the doodle so much you wanna finish it up with some lineart and coloring.
I tend to use pen and paper when I'm travelling or away from a tablet. I use pen and paper more for mind maps and first character design idea doodles than anything else.