Well... Every mix of this theme I've heard is pretty good... I think because the original is a real masterpiece... Maybe I will also join to the "Metroid II Main Theme fever" as soon as I catch a really good and different idea.
Your mix is real worth of listening to... I loved specially the harmony, all the movement through the piece... You keep the music alive in this one and that's just what I excpect from any musical piece. Congrats. Very good work!