Between these two, I'd go w/ HM901 definitely. I bought the HM901 5 months back, loved the sound, but didn't like its UI (size isn't a big deal to me since I only use them at home) and short battery life (w/ balanced amp card, the charging time is longer than the playing time!). So I sold it for an AK120, thought I'd benefit more w/ its sleek UI and longer lasting battery. But ended up I didn't enjoy using it as much as I hoped, and I always missed the sound from the sold HM901.
5 months later, I'm a happy HM901 user again, and the AK120 is gone, w/o any regret :).