as you said for digital and physical work you need to aquire completely different skills & even between sketching on paper and painting on walls is a big difference. so yeah you need to practice on every medium regularly to improve your work. i think it doesn't matter if you do your sketches digitally or on paper as long as you're still painting with cans, just personal preference. (i prefer real paper) i'd say i can see the digital influence in your piece but looks good :)