The desktop environment makes a big difference. The version number makes
some difference, but it is not as straightforward because the kernel is
growing and different things become part of the distro while others are
dropped. So making comparisons at that level is harder.
Adding a second or third desktop environment to the Ubuntu base does not
make a difference except in terms of extra disk space. Only one desktop
environment is in memory at a time. There is an exception to this. If you
are using a KDE app on Ubuntu then KDE dependencies are needed to run it
and they stay resident in memory, although they may become inactive. The
same thing happens if you run a GTK app in KDE. If you logout and switch
sessions from GNOME to XFCE then you should see a performance improvement
and it should run as fast as if no GNOME was installed. Performance will
drop as you run apps from other DEs, though.
The support for all Ubuntu distributions is the same whether it is Ubuntu,
Kubuntu, Xubuntu, Lubuntu. Mythbuntu or Ubuntu Studio. Five years will not
change. tHe only thing that will affect LTS status is upgrading to the next
version. To be LTS you cannot upgrade for two years, that is you can only
go from LTS to LTS. If you upgraded from 12.04 to 12.10 this would spoil
its LTS status. It then becomes a regular release with 18 month
support. The only way to regain LTS status would be to re-install 12.04.
The upgrade path for 12.04 would be to upgrade to 14.04 when it becomes
available, in order to be LTS. You would skip the intervening releases.