It`s a bit of a tricky one to answer, but.
The Ati card has a memory bandwidth or bus width of 4096 for a fury X model card along with HBM high bandwidth memory.
The wider the data bus the more data you can pass down it in one clock cycle of a cpu or gpu in use.
How ever HBM memory only has a maximum speed frequency @ 1000Mhz.
Where as a Nvidia GTX 1080 card has GDDR5 memory that can run at higher clock speeds meaning it can do, or process more data in a shorter amount of time.
How well each card performs because they are pretty close in graphical processing, depends on how well the game was programmed for each type of gpu, since there are slight differences in how each handles data.
It can also depend on the rest of your specifications of your system, such as the type of cpu you are using, be it an Intel solution or an Amd one. Amount of system memory you have.
Percentage wise for performance you are looking at around 10% more powerful or slightly more between a GTX 1080 card and a Fury X Ati card. The cards are pitted pretty close to each other when it comes to performance.
So it`s a tough choice to make, bar the fact a Nvidia GTX 1080 card is, or should be more expensive to buy new off the shelf.
Bang for buck the Ati fury X wins in sheer performance based on the price you pay and what you get for the money asked for the card.
The choice is really down to pricing as a major factor.