With the new RX 500s coming out next month a lot people want to know if it's worth upgrading from the RX 400s. Here is a comparison I found. In my opinion, the 570s won't really give a big increase in mining from the 470s, but it looks like the 580s will give a nice boost from the 480s.
So to sum it up, if you already have a 470 rig, I wouldn't get the 570s. But if you have anything less than the 470s or just building a new rig, getting a 570 or 580 is a good choice. You're only going to get about a 5% increase in your hashrate if i calculated right.