Which display server will Ubuntu 13.10 ship? Mir, XMir or X?
13.10 will ship with X by default. You will be able to install Mir separately for testing.
XMir isn't a display server, but rather a go-between that allows X to run on top of Mir. This allows backwards compatibility with X-only applications and allows them to run on Mir without (major) modification. If you're using both Mir and the traditional desktop; you'll be using XMir to do it.