That depends. If you're happy with 30 fps, the R9 270X will likely last through most of the next generation on its own. If you're aiming for 60 fps, then the GTX 770 would put you a lot closer next gen, and it'd be a worthwhile upgrade. The GTX 770 is a bit stronger than the R9 280X, so you'd be moving up more than a full card tier.
Having said that, the R9 270X should be performing well enough for now, so in your position I'd wait 6 months to a year until the GTX 800 series releases. Worst case scenario, the GTX 770 price drops and you get it for cheaper. Best case scenario, you'd pick up a GTX 870 for the same or lower price than the 770 is now.