They are the same GPU with just a slight over-clock and twice the memory. Glad I bought the 290X before the 390X came out because it performs basically the same. 4gb of memory is more than enough so the extra memory on the 390X just goes to waste. Over-clocking is something I do manually anyway, so the factory clock speed doesn't really matter to me. The weird thing was that the 390X became more than 100USD more expensive than the 290X, which makes me really happy having bought one before they disappeared from the market. PC components are supposed to become more affordable as technology evolves, but for the last few years this has come to a stand-still in the graphics cards market.
The slight improvement that you see in 390X reviews are thanks to the slight over-clock and mostly DRIVER OPTIMIZATIONS, something which also benefits the 290X directly but it doesn't show up in the reviews because they use numbers from old reviews with old drivers, so it looks like there's a difference in performance when it really isn't in reality.
I like that they ditched the reference coolers for the 300 series though. Not that I would ever have bought one of those myself, but those things were giving AMD a really bad reputation.