The 970's price actually went up a bit since launch and is definitely not the defacto card in that price range. Given that it's competing against the 390x (or the 390), it only has two advantages. One is that is consumes less power. If you don't have expensive electricity where you live then this isn't a factor for you. The second is a result of the first, it's a better overclocker. A 970 is a good choice if you plan on serious overclocking.
Now the advantages of the R9 390x are a bit more broad. To start, it has better DX 12 support. This will give it a boost in upcoming games. It also has double the memory and higher memory bandwidth. Games with high memory usage or those that require faster memory will benefit from this. This is already playing a role in newer games like rise of the tomb raider that require 6 GB of ram on it's highest settings. If games coming out now are benefiting we can only assume that the performance gap due to memory between the 970 and the 390x will only increase over time. It's also worth noting that because the GTX 970's last 1/2 gigabyte of RAM runs at 1/4 the speed, it suffers a large performance loss when playing games over 1080p or when vRAM usage is above 3.5 GB. Next, the R9 390x simply has more raw power. It's a larger card and has more transistors. Games that are properly optimized for it will beat out any game optimized for a 970. On top of that, because it is based on GCN architecture (what every modern AMD card is based on) it will likely have a longer support cycle than the 970. AMD will be releasing another revision of GCN this year and many more performance improvements will be handed down to every GCN card. AMD has done this last year with the Catalyst Omega drivers and again this year with Crimson drivers.