I would wait for Mir vs. Wayland winner, before making decisions. They are not ready for real world yet.

There is no decision to make. Both will need to be supported. While Mir is a very bad idea, Canonical will still ram it down everyone's throats and Steam targets Ubuntu above other distros. I'll likely write the support for at least whichever one ends up on my system and I welcome patches for either one.

Also don't forget about hardware manufacturers; There is no point in adding support for a backend which is not going to receive proprietary driver support.

Mir has already been promised support by hardware vendors, and both it and Wayland already run on the various Mesa hardware drivers. This is quite enough for GLFW to be useful.