The Axiom Pearl and Zen are fairly similar in my opinion.  The Axiom Pearl is a touch earlier and overall smoother.  You really can't go wrong with a Zen either.  If you drill them the same but change the surfaces, you can make them more different.  Honestly, both balls don't make my bag at the same time but are different enough for me to bring one or the other.  The Axiom Pearl for a smoother pearl shape and the Zen for a more angular shape.