GNOME Shell cannot be installed with Ubuntu or you break Unity and vice
versa. You need to choose one or the other and remove the one you are not
using. I am not sure if this caused your problem or just made things worse.
Both Unity and GNOME 3 are similar, but very different under the hood. They
use different window managers (Compiz for Unity and Mutter for GNOME 3).
GNOME 3 uses mostly Java up front and Unity uses lots of QT mixed with GTK.
Performance can vary. Some people find Unity faster and some find GNOME 3
faster. GNOME 3 does not work well on my equipment while Unity works fine. I
need to upgrade my Fedora partition to F15 and give GNOME 3 a fair trial
before I pass judgement on its performance because it does not work well on
my K/ Ubuntu partition.

Personally I don't like either UI and will stick with KDE. Unity and GNOME 3
require me to change the way I work and it is just too much work. My feeling
is they are toy interfaces meant for users who like smartphones and abandon
the desktop metaphor completely. For example, you cannot run GNOME 3 in
anything but full screen and drag and drop is not possible. Unity lets you
run in window but the menu is up on the top panel and you cannot run more
than one window in a workspace, so you cannot drag and drop. I have a large
monitor and drag and drop for everything. Also you cannot run anything from
the system tray as neither has one. I run Radiotray and in GNOME I like to
run Wally to rotate wallpaper since it is not built in. You cannot do this
with either Unity or GNOME 3. So KDE or XFCE get me stamp of approval and I
take a pass on both.

I am not surprised that people are having trouble upgrading from 10.10 to
11.04 with all of the changes made to the UI. I always do fresh
installations. However a nice new feature of 11.04 is that you can upgrade
from CD or DVD. It detects your previous installation and will offer to
upgrade rather than do a new installation.