RX 570 or RX 580? 4GB or 8GB?

Putting a 6 card rig together to dual mine. Have been torn between which GPU to use. Since the 470/480's are arg to find and expensive, I've decided to go with the 570/580. However trying to piece together info about these cards on the web is tough and i know everyone is going to have a different result based on their configuration.

But in general:

Which is better the 570 or 580? And does it matter if it has 4GB or 8GB memory? Does clock speed also matter?

My electricity cost is about .0777 kw/h.

Thanks for your help.