The difference is the graphics performance, you will get better quality at higher frame rates with the r9 270 over the gtx 750 ti. The only reason to go for the gtx 750 ti over the r9 270 is if you strongly prefer Nvidia GPUs (ie hate AMD), plan to use Nvidia's Shadowplay, or absolutely need Phsyx on your games.
I agree with numanator, mines a r9 290x and I can play almost any game at max so with r9 280 u could overclock it if u have a watercooler and beat my card easily . Plus the price u pay vs a Nvidia is like the smart choice unless u want to spend money.
Also that gpu will perform better than my amd cpu and its compatible