The real question is, what are you doing with it and can you really afford the 1080. I myself own a 1070, its absolutely worth buying if you want to play any games at 1080p. You don't need 144 fps in AAA titles like the Witcher 3 anyways; 60 is enough and it will handle that. Since you have a high refresh monitor I assume you play a competitive game. If you have to lower the settings a little bit so be it, but it can play most esports at 144fps. If the main monitor has G-sync of Adaptive sync you should try to match for that. the RX580 is basically equal to the 1070. Its really up to you. I will say, I got my 1070 when the 1080 was still much more expensive and now that its about 100 dollars differance its probably worth getting the 1080. 480ish for a 580 or 1070, 580ish for a 1080 although there are 1080s that are cheaper.