I never owned the S12 so I don’t know how it’s like - mostly the beamshape will be different. S12 triple emitter reflector vs S21D quadruple emitter TIR lens. S21D has different angle lenses that you can change easily. S21D is my favourite Convoy flashlight of all time, however it is quite floody even with the 10 degree lens. S12 is more throwy I guess, but like I said I never used it so I don’t know.
There is a big thread about S21D here: Convoy S21D w/ legendary Nichia 219b :) - Review & comparison w/ Nichia 519a, E21a & other lights with Nichia LEDs incl. Emisar D4V2, Convoy S21F. (Summary & measurements on P. 1)
In it you can find beamshots and more. I suggest you compare the beamshots and try to make your own decision.
They both use the same driver now so runtime performance will be identical. It’s really just a matter of which beamshape you prefer in this case.