It looks like 2015 and 2016 could be the big fight between APIs, and while DirectX 12 might reign supreme, I think that Mantle won't just roll over and die as quickly as most think. Futuremark are getting right into the middle of it, teasing its next-gen 3DMark and its "Farandole" benchmark.
Farandole will pit DirectX 12 and Mantle against each other, which will be joined by a lighting test named Balboa. Balboa will reportedly be a global illumination test, which should be interesting. Furthermore, DX12 and Mantle are both very capable APIs, with both APIs capable of delivering 7.5 times the amount of draw calls to a GPU than DirectX 11.
Computex 2015 will see the launch of the new "Balboa" lighting text, which will be rendered in DX11. After that, we will see Mantle vs. DX11 and in 2016, we'll see the big battle: DX12 versus Mantle.