The GTX 680 is in a bit of another class compared to the R9 270X. As for what way? Well, I assume you don't want a lengthy explanation on buses, clock speeds and the like, so I'm just going to say that the GTX 680 was originally designed as a very high end, enthusiast level graphics card when it first came out in 2012 for $500. The R9 270X is designed to be a more medium end, affordable GPU. Sure, it's newer, so you might be thinking it has an advantage there, but to tell the truth the R9 270X is actually a re-release of another AMD card that came out in 2012 -- the Radeon HD 7870, which was released shortly before the GTX 680 but for a price of around $350 when it first launched.
Now, the R9 270X is a bit more powerful than the HD 7870 since it's been tweaked a bit, but overall it's the same card, so essentially you're comparing a card once worth $350 card (the R9 270X) to a card once worth $500 (the GTX 680) and they were both released at roughly the same time. All that is really to say that across the board, the GTX 680 outperforms the R9 270X in pretty much every game.