It is better to wait to buy a card that is three tiers above what you already have to see any real huge benefit. Both cards are great cards. And while the 970 GTX will beat the 280X up until you get to very large resolutions, it is not worth being an "Upgrade" as it is more of a lateral move. I think that you are fine with what you have. And if what you have needs an upgrade, buy a second 280X and run them in crossfire.
Cheers!