FYI, firefox is working on switching to rendering the entire page in opengl which should bring it's motion mark score up substantially. Details on that (and how to enable it in nightly at the bottom of each post) at https://mozillagfx.wordpress.com
So Metal-only isn't really an option for many projects, it's OpenGL + Metal or just OpenGL (for now).
There will be a free OpenGL-over-Metal implementation sooner or later, so I would still go with OpenGL for a cross-platform application.