How is it different? it looks the same to me. In both cases they are providing a way for people to run existing apps on their new system.
Many Wayland advocates seem to be hellbent on trying to say that all apps will change to be Wayland only, so that they will no longer work on Xorg, but the Wayland developers seem to be more sane and are saying that the X support will need to be there forever to support apps, and that only some apps will natively support Wayland, and then only through toolkits that support systems other than Wayland.
Other than the "Mir is Evil, how dare they create something to compete with the One True Future of Display", I see no reason why Mir and Wayland are fundamentally different in how they would be supported