If you want Mantle, that's AMD only, ruling out the GTX760. You could probably find a used HD7950 cheap these days, but be careful because it may have been used for mining and worked half to death.
Radeon 7950 is older, and GTX 760 is a lewer level of graphics, compared to 270. I would go with 280 without any doubt!
Thanks! And the R9 has mantle support or whatever right because it's AMD?
Yes it supports Mantle as the other mate told you, and be aware of that mining second-hand units, because AMD is used for that purpose so often! Good luck!