Compare and contrast please; IF you could ONLY have one…..and WHY. Thank you kindly!
4x18
It has more thermal mass.
Also I don’t care for dual switch lights
Likewise, I’m sure. 
1 Thank
Thermal mass; yes, VERY important. Good point.
If you would, however, elaborate briefly on the other aspects between the two flashlights while disregarding the thermal mass aspect you reference; albeit, as important as it was. Thank you!
If you’re already ok with a large light with a handle, there is no reason to go for 4x18A over the 3x21D. The latter has more battery, more thermal mass, and a lot more throw.
2 Thanks
I’m certainly no expert on this.
The M21J uses a TIR and the 4x18 uses a Reflector.
M21J - 4x18a
Stick and stone, pipe and can; formwise
Series vs parallel
Dual vs single switch
Tir lens vs reflector
Checkout YouTube channel
@PiercingTheDarkness M21J
and @L.R.O aka FlashlighTech 4X18a
Cheers 