(Repost of summary from later discussion in case you missed it.)
A summary from my experience with the 3 lights.
M21B
Beam: Has brightest and most useful spill of the 3 lights, but less throwy. Probably as a result of the smaller thermal mass and less effective cooling, from a Turbo start I’ve found this light steps down sooner than M21F and E. Throw measured to be 280 m by Axios. Hotspot initially slightly dimmer than M21F, but once thermal regulation step-downs have set in, brightness is very similar to M21F. Overall this light is close to the M21F.
Construction & cosmetic:
No onboard USB-C charging. If you want a Convoy GT-FC with rear switch, at this point this and the smaller S11 (thanks @undefined for the correction) are the two choices.
It’s a very good looking light. Smallest of the 3 lights and pocketable. Knurling is not as deep as Emisar for example, but at $30, I am NOT complaining about anything.
I would highly recommend to change rear switch to metal switch from Convoy as it makes the light very pretty and more importantly, makes the click (full press) and tap (half press) much easier, for me anyway.
UI: The well known Convoy 12 Groups. I prefer Anduril but whether 12 Groups is a plus or minus is of course personal preference.
No Anduril means you cannot play with settings such as thermal regulation’s max T (factory setting is 55 C per Simon). Judging by the result (similar step-downs to other lights I have), maybe there is no need.
M21F
Beam: As these GT-FC Convoys get larger, spill becomes dimmer and hotspot becomes brighter. The M21F has a slightly brighter hotspot than M21B. Overall this is a similar light to M21B.
Construction & cosmetic:
Among first Convoy lights with onboard USB-C charging. A very good looking light. Top end is not anodized and not smooth to turn the first few times but now is better.
The side-switch: it “clicks” very nicely, but is hard to press, so not my favorite. My firsts 21F’s switch has a dead zone and needs to be clicked at dead center, which is not always possible. Simon immediately sent a second light without charge and its switch is good.
The side switch is also in same position as the charging port, so I would definitely recommend screw-on or clip-on Convoy clip for example at minus 90 degrees from the switch to locate it quickly.
UI: Still no Anduril, just Convoy UI for Side-Switch lights. Personally I like this UI and prefer it to the 12 Groups.
M21E
Beam: Brightest hotspot, best throw, and dimmest spill of the 3 light, but the spill is still bright and functional. Meaning the beam is balanced and like the other 2 Convoy’s makes for great all around multi-function, multi-use EDC light. Larger light has more thermal mass and better cooling, and this means for example step-downs in this light occur a couple minutes later than M21B and F.
Construction & cosmetic: I am not too thrilled with those diagonal grooves at the head of the light but otherwise it looks good. This size fits more easily in jacket pocket, unless you have loose pants with side pocket (not jeans’ front pocket). I do carry it in side pocket of my pants.
The side switch is same as M21F: Beautiful but short travel and hard to press. This means single click is easy, but multiple clicks are not. OTOH the good thing about this is it protects against accidental activation of the light, which is not a good thing in this high-power light.
UI: Same as M21F, UI for Convoy Side-Switch lights.
IMHO the combination of smallish size and USB-C charging makes the M21F sort of the sweet spot of the 3. OTOH because it is so rare to find a bright throwy light with world class CRI and no green tint, I like the M21E best. Its disadvantage is that it is also biggest and heaviest, and that large top is unwieldy. Lastly, if you like floody light and tail switch, and don’t mind the lack of on-board USB-C charging, then M21B is the one. One of these GT-FC40 4500k plus a Convoy S21D with 219b 4500k would make an incredibly inexpensive and fantastic start/back bone for any light collection with accurate color rendition in mind. You can’t buy anything better even if you want to spend more money IMHO.