The default desktop will be Unity 7 and it will be powered by an X.Org Server when running Ubuntu 13.10. However, the Qt5-based Unity 8 in conjunction with Mir will be readily packaged and available for those wishing to give it a go. Meanwhile, for the Ubuntu 13.10 state of the phone/tablet version, that is expected to be Mir-powered in time.
The Unity 8 + Mir option for Ubuntu 13.10 is to be discussed during the final day of this week's virtual Ubuntu Developer Summit. The session abstract explains, "As part of the convergence story, Unity 8 running on Mir needs to be made available as an alternative session on the desktop as soon as practical. While not targeted as a production-quality release in this cycle, getting a functioning system into the hands of designers, engineers, and some plucky users at an early stage will help iron out wrinkles before they become problems that affect other form factors."
Unity 8, a.k.a. "Unity-Next", with Mir has already been shown off running on an Apple MacBook Pro. On the mobile side, Mir/Unity-8 has been shown with a Google Nexus 4.