http://www.anandtech.com/bench/product/1746?vs=1748 Well out of the box the 390x is faster but it uses almost twice the power. The 480 has all the latest tech that GCN has to offer. The question is, "what do you value more?" Efficiency and latest tech or raw performance?
R9 390x is faster. Whether it's better or not depends on your needs though. While efficiency and powersavings is attractive, it means little if you oc your system to the max. I would not go for the 480, instead I would wait if possible for a 490 that will most likely be a real upgrade. The 480 comes close to an R9 390, which in turn comes close to an R9 390X. If money is an issue though, then the 480 is a decent upgrade from your current card.
What res monitor do you use. 480 is fine for 1080p, but not powerful enough for 1440p. Where as the 390x can do 1440p.
The 390x is clearly faster than RX480. But Rx 480 is more efficient due to 14nm FinFet and polaris architeure to know more about RX480 and R9 390 Check this out.
I'm not trying to hijack the op post, but given the future games developed with vulkan and dx 12, which one is better?