Power usage is not the same. If you are not paying for power then 390 are the best. Else the RX470 is better. Which brand did you get? I read that msi and PowerColor is highest sols if you do the memory mod
I got rx470's from xfx and if i will buy r9 390's from sapphire nitro. 1 rig will run at my home other is probably gonna run at university's lab up to May 20. So I'm thinking about placing r9 390's to university since electricity is free and they will give nice profit and also teachers will be happy about showing it since no body in the school isn't know neither blockchains nor crypto currencies. And also I'm curious about psu for r9 390's psu or psu's gonna cost me too much i guess.
I still suggest going for the rx over the R9. As it has a bigger change of be useful in the future if you are forced to mine at home. Your lectures might not be so happy with the idea if they feel the heat the R9 will produce.