The discussion above does a good job comparing these two flashlights. I don't want to repeat the things that were already said, so I am adding just two or three new things that have not been mentioned so far.
Most of the reviews I have read, including the one by selfbuilt, say that the
Nitecore MH20 outputs substantially more lumens on its middle three modes than it is rated for. Here are the numbers from selfbuilt's review.
His sample, at least, jumps from 1 lumen in "lower" mode to 125 in low mode. That's a big jump. At the top end, selfbuilt reports the opposite. Mode spacing on the top three modes is too small. The difference between medium (at 370 lumens) and high (at 590 lumens) is only 220 lumens. That's much less than even a factor of 2. With turbo at 990 lumens, the difference between high and turbo is also less than a factor of 2.
Other reviewers and CPF members have reported similar findings. The only exception is the review by MHanlen that was posted yesterday. He measured low at 73 lumens, which is still a pretty big jump from the 1-lumen "lower" setting. At the top, MHanlen reports 301, 525, and 928 lumens.
If these numbers are right, then my strong opinion is that
the ZebraLight SC600 Mk. III has far better mode spacing than the Nitecore MH20. That matters a lot to me. Compared to some of the more subtle features that were compared above, mode spacing is a biggie.
Another difference that has not been discussed yet is the drivers. Nitecore uses a buck driver in the MH20. ZebraLight uses a boost driver in the SC600 Mk. III. On all modes below turbo, output on the MH20 enters a declining, direct-drive-like pattern somewhere around 75% or 80% of the way through a run. On the ZebraLight, output stays nearly flat, until a forced step-down occurs when the battery gets low.
Which of these you prefer may be a matter of personal choice. For the ways I use a flashlight, I like the predictable output levels given by the ZebraLight. I like flat output.
For those concerned about accidentally over-discharging a Li-ion cell, the ZebraLight has small edge over the Nitecore. Like all recent Li-ion models from ZebraLight, the SC600 Mk. III has a low-voltage cutoff that turns the flashlight off when its battery gets low. Cutoff occurs at 2.7 volts. On the MH20, the main beam blinks to alert you when the battery gets low. The Nitecore does not have a low-voltage cutoff, so you could get into trouble if your MH20 were accidentally activated or left unattended. It gives you a good warning, however, if you are there to notice it.