There is a 120$ to 150$ gap between these two cards. The fury is about 10% faster than the 390x. With more room for overclocking the 390x would make the difference go into single digit percentages. As well the 390x has double the vram although for now 4 gig (of HBM) should work fine.
I've been impressed the way the fury has nicely positioned itself between the gtx 980 and 980ti(even coming close to the 980 ti in some cases). Although, it's bothered me how much cheaper the 390x is. I plan to game at 1440p with freesync. Which gpu should I buy?