Stock lenses on my Emisar’s do not have AR coating. That said the beam of D4V2 with 219b 4500k is equally as beautiful as S21D’s; the 2 look the same to my eyes. Duv actually measures nearly same.
Basically you can’t go wrong with either light. Some comparison numbers:
Duv
S21D ~ –0.0084
D4V2 ~ –0.0087
Basically both measure in the mid 80’s for Duv.
Throw
S21D 170 m with 10° Clear, 157 (10° Bead), 144 (30°), 113 (60°)
D4V2 137 m with Carlo 10622, 140 w/ 10621
If you want throw the S21D is better with the 10° clear TIR.
Output
About 1700 lm for both using Texas Ace Lumen Tube.
Sustained Brightness in favor of Convoy (that famous 37% at 10 min) at the expense of heat. I plan to do a comparison graph later when I’m not in lazy mood.
Size
S21D is larger so it has thermal mass advantage related to specific heat, but the smaller D4V2 feels GREAT in hands. The higher thermal mass (without battery S21D 4 oz, D4V2 2.7 oz) is important for sustained brightness and this and the longer throw of S21D with 10° TIR are the 2 main reasons I personally prefer S21D, but YMMV.
Drivers, Cosmetic, Cost, USB charging
Convoy has 8A Buck, D4V2/D4K has Boost option but that costs $12 extra.
Emisar is prettier to my eyes, with multiple choices for finish/color.
Emisar D4K with boost driver I think is about about twice the cost of the $30 Convoy S21D.
New Convoy will have integrated USB-C.
All IMHO/YMMV. You could do what I did and ordered both Emisar and Convoy. 
