You should definitely go for a r7-260x as it is an overclocked 7790(more clock on gpu and mem.) + it has TRUE AUDIO.
The only reason why i would ditch the r7-260x and go with the 7790 dual-x is for the better looks and asthetics(just in my opinion).
But I think you should go with r7-260x(if asthetics doesnt appeal to you that much).
[note: I personally am deciding to buy a 7790 dualx if it is available cause it would look good with my mobo: ga-78lmt usb3 v5.0]