Love coming across sketchbooks that have loads of study in them - gets me all inspired! I'd say thus far I love your oil studies - they have something about them that your digital works lack. Might be in the way you do them - digital is horrible for being able to just get in there and noodle with details before you should, yet your oils have this great sense of freshness of brushstroke. Could be worth paying conscious attention the next time you do an oil study how you put down the strokes, and try and replicate that with your digital stuff.
And on a random note, I love the really dark red fingers on your latest teen challenge piece - something about that coupled with the frost subject that tells a story for me.