Nvidia’s 970 is the only card that succumbs to AMD’s flagship [390x] in certain situations.
(Though since the prices are about the same, no need to get the 290x)The Radeon R9 390X with its stock overclock runs Assassin’s Creed Unity faster than an overclocked GeForce GTX 970, but doesn’t really stand out against the 290X in spite of its doubled on-board memory.
AMD’s Radeon R9 390X doesn’t only get an overclock, it also boasts two times the GDDR5 memory. It’s the latter, not the former, that helps AMD catch an overclocked GeForce GTX 980, which it even beats in places.
The Radeon R9 390X’s 8GB are a great fit for 4K though, no question. You only need this much memory at lower resolutions if you’re installing high-def textures and demanding mods.