Either count will perform much better than KO or Hi-Lo, assuming you can handle the ace side count. I personally use HI-Opt II and sims have shown it to be slightly stronger, but there is little difference. Omega II can be found in BlackJack For Blook. Bryce Carlson's book is well-written and he has a great knack for mixing a little humor into the text to keep it readable.
Sims show that while H02 is superior in shoes, AOII has a small edge in pitch games. Now, much more shoe games than pitch games exist in the world, but counters using a level 2 system are more likely to go for pitch.
This is of course, major hair-splitting stuff.
Hi-Opt II and Omega II are both the "hot set-up" when used with a side Ace count. With no side counts, I've found the Mentor Count to be the highest performing all-around (hand held or shoe) level two count.
With Zen the Ace is not counted as 0, it it also counted as -1 as with the Mentor count. One other difference between the two is Zen counts the 9 as 0 and Mentor counts the 9 as -1. Maybe that's what you meant. You confused the Ace with the 9.
I agree, Mentor is probably a little stronger. I Don't have any sims to back that up. All I have are the sims fred renzy ran and have published in his book. They show Mentor to be pretty equal or slightly stronger then Zen.