RX 470 has proven to be more energy efficient in terms of energy consumed vs reward, so I would suggest go for 5 RX470.
Agreed! Not sure why anyone would opt for 480's vs 470's at this point. My modded MSI 470's can get over 28 on ETH, cost $40-$60 less than 480's and consume significantly less electricity than the 480s. Best stable HR you will probably eek out with a modded 480 is around 30, so that extra 1-2 vs a 470 will simply never be worth it due to the excess cost up front, but more so due to the extra watts needed to run 480s. Certainly if you already have 480s in your rigs there is no point in switching to 470s, but for new setups it really seems like a no-brainer between the 2.
For the better memory overclocking is really the only reason. Also if you decide to get out of mining later they will sell easier and quicker. You can under volt the RX 480 as low as an RX 470 with WattTool.