And the same way XWayland could be faster than X for particular apps it's likely that, assuming Mir will be faster than X, XMir will be faster than X for particular apps. Whole desktops are out of scope for XMir and XWayland for common sense. There is no reason to not use X if you are going to use an X desktop, since you will be unable to load Mir or Wayland apps.
Over Wayland? It achieves this using a library developed for Wayland.* Supposed to get Android driver support
¿?* A seemingly more devoted team
Both have a fully open source license. In fact, it's easier (i.e., you don't need anyone's permission) to fork Wayland on a closed source project. With Mir you can, if Canonical allows you, because of the CLA, but you can't without their permission, because of the GPL license.* A fully open source license
I don't know what is this based in. Any of the two might be the lighter one. Wayland because of being just the protocol, or Mir because of only implementing what Unity needs, instead of having to be usable for the whole ecosystem.* Probably will be more light-weight in the end
Brief version: 13.10 will ship Mir by default, with Unity 7 running on XMir.
I wonder how do people manage to turn that into "Canonical will not ship Mir until 14.10", when it clearly says it will ship it (READ THE TITLE) in 13.10, and will use Unity 8 on 14.04, which runs natively on Mir.