It doesn't matter because future next gen games don't run on XB1X, they wont be compatible unlike the with the Series S which is the entire point of the discussion.
Also Series S uses GDDR6, XB1X uses GDDR5.
Which is inconsequential. Memory bandwidth is determined by the speed of the memory and the bus width. The Series S only has a 128 bit memory bus. The One X has 384 bit memory bus. The GDDR6 has a serious handicap due to the narrow bus. It's literally the same memory as the Series X but the Series X has a 320 bit bus on 10 of the 16GB of memory it has which results in a 560GB/s memory bandwidth. The other 6GB of the Series X memory only has 336GB/s memory bandwidth because it's stuck on a 192 bit bus.