I decided to go with 24GB / 3090's, although if there had been a suitable 16GB alternative I would have taken that. 12GB feels a little too tight for my work. Having said that, I don't think I'm in the middle of the distribution as a C4D user: visualisation work, high poly counts, high res textures, environments, displacements, volumes etc. They are all memory hungry.
While I doubt I'll need it - for the foreseeable future at least - the option that does become available with the 3090's, but not with the lower spec cards, is that of NVlink - giving a potential pool of 48GB.