The difference between the RX 470/480 is the memory clock speed and typically better cooling on the RX 480/580. The RX 470 has around memory clock speed of around 7000 MHz or less and the RX 480 uses a 8000 Mhz memory clock. As I said earlier the cheaper RX 470 will have a better chance of using a lower tier memory brand. After bios mods, the difference in hash rates is around 5% - 10% between the two. If you can get the RX 480/580 for around the same price, go with that.
Thanks again, you are really very helpful!
I take the chance to ask another question: what program do you use to mine, if I may ask? I was looking at claymore's one, which appears to be the best one. Do you also think that double mining (eth+sia) is good or just is not worthed (maybe about earnings/hash decrease/life og gpu)?