The Convoy S21A is available with the SFT40, but i can find the S21B only with the SST40.
The S21A and B are almost identical, A has a pill and the B has a shelf. My A models seem to take a few seconds (15+) longer to heat up so i can feel it in my hand but that is probably due to the different LED's more than anything else. The weight is almost identical at 86 g and 89 g respectively so the mass to absorb the heat is relatively insignificant. And that is with both driven at identical amps - I measured.
The S21B with the CULPM1.TG emitter is a super thrower and I can highly recommend it, if that's what you decide, but be sure to get it with an OP reflector or you will see some very ugly lobsided rings around the tight hotspot.... these go away with the OP reflector. I speak from experience.
And if you were wondering, the S21A and B can swap reflectors.
I will be ordering a red body color S21A with the SFT40 soon.... Link below to Simon's Aliexpress store: