Wait until the final product is out regarding dx12. Trust nothing Microsoft touts. Mantle can still end up being more efficient.
Mantle might well be on GCN hardware, but it was built for that. Based on the Star Swarm benchmark DX12 performs pretty similar to Mantle and it's not even finished yet, though to be fair to Mantle it also isn't finished yet.
we honestly need both Mantle and Dx to be relative in performance to each other, as they will feed off each other for better efficency resulting in frequent performance improvment.
I don't think we will see many, if any games use both DX12 and Mantle though. DX12 and GLNext is possible though.
You mean Vulkan ( new name for OpenGLnext ) ... http://community.amd.com/community/...blog/2015/03/03/one-of-mantles-futures-vulkan
