Ubuntu-GNOME vs Ubuntu-Unity
Ubuntu with Unity is the standard version of Ubuntu distributed by Canonical (the company that created Ubuntu).
Ubuntu with GNOME 3 is not currently supported by Canonical but many community members are interested in it and/or have put time in to make it work. Using bittorrent, a group of users are distributing a version of Ubuntu with GNOME 3 called gNatty GNOME
Unity depends on many components from GNOME but doesn't look like GNOME. So the look and feel (the user experience) of GNOME and Unity compete with each other.
GNOME 3 doesn't provide a "minimize" button by default. Because it has not been the focus of Ubuntu's developers, GNOME 3 may be more buggy on Ubuntu. (Which could make Fedora an attractive option to GNOME 3 fans).
Both GNOME 3 and Unity represent a big change from the GNOME 2 interface, and many users must learn new ways of getting work done if they make the switch from GNOME 2.