I am using Debian 6 .I would like to install any of Unity or Gnome 3 desktop Environments .
This is not likely to ever be readily available.
However, if you're serious about wanting Gnome 3 you may as well make the jump to run Wheezy and install it from experimental.
See Raphael Hertzog's article about Gnome 3 on Debian (with instructions).
You will need to install aptitude and tasksel before install gnome
Aptitude is an Ncurses based FrontEnd to Apt, the debian package manager.
Tasksel is a tool that installs multiple related packages as a co-ordinated “task” onto your system.
# apt-get install aptitude tasksel
Install gnome on debian
# tasksel install gnome-desktop --new-install
Questions like this could easily start a flame war, so I'll be carful with my answers:
Resources: As the choice of which GUIs [Graphical User Interfaces] is used seemingly is widely based upon taste, performance seems to be less discussed: In my experience, whilst working on a Netbook, both Unity and GNOME3 use a lot of system resources [I made the same experience with KDE too]. As I am quite limited in this environment, I had to look for something lighter.
Pling?: Luckily, there is a huge field between those who laugh about a GUI other than a command line and something like GNOME3 or the Unity desktop. On this field, different takes on how a GUI should [not] work and what should [not] be included exist. And all of them are highly customizable in their own ways.
Choice: Linux leaves loads of choices to the users. On Linux Mint Debian Edition I switched from MATE [also an alternative to GNOME] to XFCE first and then to LXDE. Whithout any losses or trouble worth mentioning. Some applications need a certain GUIs to work properly as it seems to me, but then there is most likely an alternative to work you way around this issues.
My point? Try out, experiment, and always ask yourself: Do I need this|that?
My suggestion: Try a Live version of a Gnu/Linux OS with GNOME3 or Unity. Or XFCE. Or LXDE :)
About unity, at this time please do not install at Debian. A request to packaging to Debian has been submitted.