The rule of thumb (no pun) is thumb handles the bass strings when finger playing. When strumming, it's best to use a pick (hence the term--flatpicking), The guitar will sound brighter with the stronger attack from the pick, and i think the sound will be more uniformly consistent, a good quality if you are a rhythm player. When finger playing, the dynamics can change depending on how you attack the strings---with the fleshy part of your thumb, versus the nail, etc. To me, finger playing (without finger picks) yields a soft, warm tone, whereas the finger picks tend to make the notes sound brighter, though a guitarist with good dynamics could also replicate a soft warm tone with the finger picks. It's a preference to the player, and what song you are playing.