I have had 1 Taylor with the first gen AA ES and 3 with the second gen 9V ES. Like others have said, there is big difference between the two, and I didn't like the AA ES at all. I have had no issues with any of my Taylors with the second gen ES, and I can only imagine that the third gen is even better. If I ever have to send my 614ce in for a little love from Taylor, I will for sure have them install the newest ES they have at that time. Until that time though, I am quite happy with what I have.
I would have to be getting a crazy good deal on a Taylor with the AA ES before I would go back to it again, because it would go straight to Taylor first thing for an upgrade. I had a 714ce that I just loved that had the AA version, but it just had a horrible tinny sound plugged in that I couldn't get to go away. We parted ways after I played it live a few times. I had a friend play it for me while I was doing a sound check, and it just made me sick to listen to. It sounded nothing like the 714 at all. I was so disappointed.