Depends on your budget, GTX770 is much faster than the 270X, but also more expensive.
The R9 270 (non-X) is a more popular card because of its almost identical to R9 270X performance at a lower price, it's probably the best price/performance you can get.
The GTX770 is also a great price/performance card, it's just the price/performance card of a higher class of gpus.
it depends on how much you value the quality of games. if you love the max settings, then the 770 is worth it. if you are fine with good settings, but not quite the best, then the 760 is fine.
gtx 770 is better than the gtx 760, but its not so far ahead of the r9 280x because different games that works better on specific cards. the r9 290x and gtx 780ti will just make your day.