Hash rate is almost identical. However IMO, the RX 580 is a better card than the RX 480. Most have better cooling and the power draw issues from the early RX 480's was also addressed. In regards to the DAG epoch change. My reported hash for seven cards went from 202.7 to 202.2 MH/s. Even if the hash were to drop by 10% over the next 6 months, that can easily be addressed by adding another RX 570 4 GB card for ~$200. At that price point and the much better dual mining performance over Nvidia cards, still makes the AMD RX cards easily the best choice for ETH.
The problem is you will want to switch them out 4-6 months in the future or mine something else if it is even profitable, because they will become too slow eventually.
So either get some gtx 1060/1070 1080s or wait on the incoming flood of cheap rx cards as people panic. But Nvidia is planning on releasing a mining gpu so the market could come completely saturated with gpus in the coming months. New people buying old rx cards plus farms switching over to mining gpu = perfect storm.
Proceed with caution