You mean between the SC62w and the SC63w? Lots technically, probably not a lot in actual use. Underneath, Zebralight mas moved to the XHP-class emitters which offer more output, but they also require a more sophisticated boost driver for proper operation. Zebralight has been building great boost drivers for years so this is just a normal progression for them and all indications are that they got it right, but its complicated and expensive electronics. For the end user, the result is about 20% more light output on the highest level, which is noticeable when held side-to-side, but not really a huge increase since our eyes don't detect light intensity in a linear fashion. Another minor difference is that the XHP35 in the newer light is slightly larger, so assuming no reflector changes it should be slightly floodier, or at least have a larger hotspot, than the SC62w's XM-L2 emitter. And then of course there's the battery fitment debate which I won't rehash here, but the newer light has tighter tolerances that make it a bit more picky about which batteries fit inside.
The bottom line, however, is that the older light is discontinued so if you want a small, 18650-powered Zebralight you really only have one choice - the new one. Unless you poke around and can buy an SC62w second-hand, of course.