Same difference. There may be a negligible difference in the 20G shot starting as a 'taller' but 'less wide' cylinder, but by the time the shot is at the target, it will have spread beyond measure.
Being less prevalent, the 20G cartridges will likely cost more. Also the 20G gun, being lighter, will absorb less recoil. Funnily enough I've had discussions with shooters saying they like the 'lesser recoil' of the 20G, but I can't wrap my head around that. Might as well ask, as maybe I'm missing something.
With 24 grams of shot being expelled at the same velocity, the forces pushing 'back' will be the same in each, but the heavier gun won't move as much as the lighter gun, right? Something about the laws of physics. Now I'm no scientist by any means, but I suspect the shooters who feel the 20G recoils less than their 12G, probably have a better "fit" with their 20G than they do with their 12G. Or am I overlooking other things?