if you are talking about the price r9 270x around $220 and GTX 760 $260.
The speed difference will depend on games you are playing, there are games that they have almost the same performance but there are games that gtx 760 is ahead by 5-10 FPS, r9 270x doesn't get ahead with gtx 760, IDK when mantle comes if it can win or atleast match the performance of the more expensive gtx 760.