My answer is known here, but I'll repeat for some of the newcommers - UNLESS you use proficiently a secondary count consisting of A-2 vs 2&5+1, or similar, you will not realize the extra theoretical gain that AO2 portends. Ace density estimate per 1/4D as proposed by Humble and Carlson and Uston and Revere will NOT cut it. SO you wind up working harder to get the same gain as ZEN w/o Ace side-count. AND if your Ace 1/4 deck density estimate is not up to speed you won't even get your ZEN's worth.
Let me see if I got you right:
1. You are saying that the Zen count (without count aces) is as much powerful as AO2 (whith count aces)??
2. I'm learning the AO2 but due to this fact I'm considering to switch to zen. I can count deck in 25 sec, TC is no problem. Would you advise me to switch to Zen and if I do should a be keeping a side count of any other card?
I'll play with 6-deck games for now, because there nothing else in my country