Which would you choose?
470 with 8 gb for sure. Got few of them.. ( Sapphire ), flashed and all are still working great (knock knock on the wood). The best card in my opinion for mining is still RX 480 8gb (red devil). Before the DAG increase it did 30 mh/s on ETH, with this DAG it's still doing 27,6.. On other coins with lower DAG it does 30-31 mh/s with 90W from the wall.
What about dual mining? Does the 5xx series pull ahead?
To the best of my knowledge they should be more or less dead even if both are tweaked and modded. If anything the 500 series just uses more power at default settings and is a bigger pain in the arse to get 6+ GPUs working on Windows with drivers etc.... I'd buy Rx 470s every single time if I could get equal access to both of them at a similar price.