I'm getting one of the following soon:

http://www.overclockers.co.uk/showproduct.php?prodid=GX-225-MS
http://www.scan.co.uk/products/2gb-asus-gtx-770-directcu-ii-oc-28nm-pcie-30-7010mhz-gddr5-gpu-1058mhz-boost-1110mhz-cores-1536-dpor

I can't decide which one to go with.

AMD with mantle or nvidia with free games & solid drivers?

Either of these will handle modern video games at very high resolutions and framerates. It then becomes a matter of preference. Myself I prefer nVidia gear, but that is because I run mostly Linux and they have better drivers for Linux currently than AMD. Also, if you want to do some serious math processing, the CUDA cores of the nVidia cards is totally awesome, and they have very good support for programming them.