I have a 290 that unlocked to an X and the difference in Unigine Valley was about 5-6 fps. The X version is also a lot smoother at keeping 60 fps when playing Crysis 3.
People say there isn't much of a difference, and you can just overclock the non X but when you can actually compare the two side by side it is a different story. You are overclocking one card to match another, but that card can overclock as well. Once games are more optimised for these two cards the X will justify it's price premium in the same way the 7970 did. Get the best card you can afford.
I also suggest the Sapphire Tri-X version as well.