GTX 760 or R9 280 DUAL-X

Hey guys,

Im looking at upgrading my ageing GTX 560ti but on a very tight budget.

I have the chance of a SAPPHIRE Radeon R9 280 DUAL-X OC for £50 or an MSi Nvidia GTX 760 2gb for £70 (both second hand).

Which one would be the better upgrade, current system is:
PSU - Corsair AX 760
MB - Gigabyte Z97X-Gaming 7
CPU - i7-4790K
GPU - Nvidea GTX 560 ti
2X4gb HyperX beast 2400 ram

The 280 is the faster card.

http://www.anandtech.com/bench/product/1332?vs=1038

http://www.redgamingtech.com/amd-ra...eforce-gtx-760-review-analysis-tech-tribunal/

Thank you