if u looking between 980ti with the hybrid one with the fury x, that should be decent comparison...
980ti have more vram compare to fury x... 6G compare to 4G...
imo, if u're looking to get a good 4k exp, a higher vram should be the solution..
but, also depend on what u gonna use the gpu for...
fury x does have higher Memory Interface bus... more 10x faster than the 980ti...
this should be important for media contain creator... not sure for gaming though...