Can the PCV do a "learn mode" on an FZ6 with an O2 sensor? Is that worth considering? All options in that arena (PCIII, Dyno, PCV, ECU Unleashed. Any others?) seem to be comparably priced. If you could do JUST the PCV it would save a little $, but still wouldn't be as good as a PCV plus dyno, I don't believe.
Like turbid said, it should boil down to what's available at what price with how reasonable or motivated a seller. I like my S1 a lot. I've ridden an S2 and I couldn't say which I liked better. The S2 felt smoother, but was younger, so that's expected. Given the age difference, I'm not sure I could tell any difference, apart from the braking. Maybe the throttle on mine feels a little less "tame"? Anybody else rode both?