I started by thinking the new one, and the more I look at them, and the more confusing I get... But I'll stick with my first impression, so the new one (because at the end of the day, thumbnail are here to trigger immediate reactions most of the time.)
The new one looks clearer, has more movements (the back line), and a more mature vibe overall (still a chubby baby face, but with clothes and emotions).
So the new one feels like a slightly older version of the old one... (new, older than old.. it's twisting).
The old one has more personality in the art though, less of that cold digital perfection. So maybe do the new one using your old one method? (action lines, clothes, emotions...)