X11 is the technically superior choice. #getoffmylawn
Still, from that point of view, the Mir thing has been a success for all those itching to replace a stable, mature, well known and tested, versatile, and powerful windowing system with a new and untested stripped down windowing system simply because they don't understand why someone would want some of the features X11 has, and are under the impression it's bloated because it's bigger than Windows 2.0 was in 1989.
Mir has helped create the illusion the decision has been made already. We are transitioning, no more debate is needed (or will be accepted) as to whether we should, and the question is what we should transition to.
Much the same mistake was made with GNOME 2 to GNOME 3, a transition that Ubuntu helped along in the same way with Unity. Users rebelled, with forks like Mint attempting to roll back the damage, but the end result was a deterioration in the perception of GNU/Linux as a potential replacement for Windows. Distributions based upon GNOME 3 and Unity got the "slick", "professional", treatment, with users finding fast that it wasn't what they actually wanted. The GNOME 2 hold-outs didn't have the resources to ensure GNOME 2's forks had the same level of support, and so ended up with systems that looked to new users dated and ugly.
We will see the same with Mir/Wayland, except worse. We'll have five to ten years of having to deal with an immature windowing system that, by the end of the process, has just as many hacks and quirks as X11 but will almost certainly still lack key features X11 offers. X11 holdouts will find themselves using an increasingly unreliable and unstable platform as newer hardware requires new device drivers, without the level of support needed within the X.org X11 community to support them.
We're all going to lose. The best free software users can hope for now is that Google continues to extend Android to eventually offer a decent desktop experience. I don't know why they would, perhaps to replace ChromeOS, but at least you're looking at something mature there. But that's not here now, and the next five years will be rough for GNU/Linux users. We'll likely be as mainstream as FreeBSD by the end of it.