They are pretty close, a mixed bag depending on which game you look at.
See some tests here > http://www.eurogamer.net/articles/digitalfoundry-2017-radeon-rx-580-570-review.
I'd personally take the RX 570 Gaming X, specifically because it has a good amount of VRAM (below 4GB will start pushing the boundaries). Digital foundry apparently had a great time overclocking them too if that is something you do.
Quote > "The RX 570 is the real star of the show in terms of overclocking. The 7gbps GDDR5 memory runs perfectly happily at 8gbps, while the Asus Strix's 1300MHz core (already running 66MHz over reference spec) easily made the leap to 1400MHz. We re-benched three titles and found the 570 to outperform the reference RX 480."
Hope this helps