some basic info compared:
Olight S1 Mini High CRI: 54mm x 21mm, lowest mode is 0.5 lumens
Fenix E16: 51mm x 21mm, lowest mode is 3 lumens
both lights have tail magnets, both have clips that work on a hat, both have strobe, both have electronic lockout option, both have last mode memory
Fenix has XP-L Hi Neutral White option, Olight has XP-G3 High CRI option @5500k
both have Low Voltage warning
Fenix: When the voltage level drops below the preset level, the flashlight is programmed to downshift to a lower brightness level until Low output is reached. When this happens in Low output mode, the flashlight blinks three times every five minutes to remind you to replace the battery
Olight has a red led that turns on in the middle of the power button, as a low battery indicator. Olight does not step down to moonlight on Low battery, it will still work on other modes until battery protection is tripped.
both Fenix E16 and Olight S1 Mini weigh the same 22 grams empty.
The bezel is not removable on the Olight (no lip, it is inset into the body, removal is destructive), so LED swap is unlikely.
The Fenix UI is hold for ON, Hold for Off, click to advance modes, strobe is press hold for 1.5 seconds
The Olight UI is Click ON, Click Off, hold to advance modes, strobe is triple click
I prefer the Olight UI, I prefer that the Olight has a Lower Low. The Olight costs $10 more than the Fenix, which costs $40. (Olight can sometimes be had for 20-30% off, PM me for details)
If the Fenix can be modded to Nichia, that is a big advantage. I also like that the Fenix tailcap allows connecting a keyring.