16 or 20 is still a low overall multiplier so assuming we expect SSD to mitigate that, is it likely to make a significant meaningful difference which it is? 16 is obviously easier architecturally to support.
16GB will probably have more than twice as much memory available for the game when compared to the 12GB in the X, something like 4 to 5x the amount available for the PS4, and that's still pre-loading/caching quite a bunch of stuff.
A pretty inordinate amount of memory was used for pre-loading/caching, frequently more than half of the available RAM.