I have a H600w Mk I, and an H600w MkII.
Compared to the Mk I, the Mk II has better anodizing, a better UI (esp. for strobe, which I use on my bike), and slightly (trivially) better output.
The Mk III offers: what exactly? Going from the comparison sheet, it looks like:
1) A better CRI, which is good.
2) "brighter," though no numbers yet
3) "more efficient," which is always good, though my guess it that the improvement will be small
4) "wider input ... voltage range". If the input range allows you to run a pair of DL123 primaries as well as the 18650, that would be a very handy option to have.
5) "wider input *and output* voltage range" Why "wider output voltage range"? Why would the LED want to see a range of voltages coming to it from the driver?? I thought the emitter had a preferred operating voltage, and the only thing that should vary is the current?
I wish they were releasing the non-floody version first, as I prefer the clear lens. That way I can add my own DC fix when I like, and have a bit more throw when I like. It will be a tough thing to hold off on buying the floody version. Must...resist...hold out...for clear lens!