SST-20
* Fairly comparable throw to XPL HI. Possibly slightly thrower.
* Greenish tint especially at intermediate and low power settings.
* Lower vF (forward voltage). Pulls more current and runs significantly hotter than XPL HI. High heat means you can't run it as long as XPL HI at higher settings. Also less output means you have to run it at higher power to get a comparable amount of light.
* 4000K version is available in high-CRI with good R9. With this version color rendition at higher brightness settings is excellent.
* Cheaper. But is it really worth saving $10 to get an inferior light?
XPL HI
* Significantly brighter at all power settings. And while initial turbo on a fresh cell might look similar, the SST-20 rapidly becomes dimmer much faster due to overheating and faster battery drain.
* Runs cooler due to higher vF
* Battery lasts slightly longer due to higher vF
* Much superior rosy tint. No ugly green like SST-20. Even at low brightness settings.
* Wider beam
* Costs more
I don't personally own a D4 with 5000K SST-20, but I do own 4000K SST-20 and have compared it to 4000K and 5000K XPL-HI.
Personally, I feel the XPL HI is enormously superior to SST-20. The only time SST-20 is worth it is if you're getting the 4000K version specifically because you want 95 CRI. Even then, I still often find XPL HI to be the superior choice since it puts out a LOT more light, has much better tint (not greenish), and doesn't get as hot as fast.