They're nearly identical in difficulty, from my experience. In favor of Zen: you can ignore all 9s. In favor of Mentor: 3 has weight 2 and 9 has weight -1, which means that more 2-card hands cancel - or at least they do in my head - because you have the same number of +2 cards as -2 cards in the deck, as well as an equal weight of -1 and + 1 cards. Whenever I play with another count and come back to Mentor, I see cancellations all over the place. Halves or Zen tags with a 2DTC conversion would be an awesome system.