Difference between Windows defender and Spybot search n Destroy

Can anyone here me give the difference between the Windows defender and Spybot search as this is necessary because both are the important products from the security point of view as well as from the practical knowledge, any information related to this would be added information for me,Thank you!

Microsoft Windows Defender beta 2
Min processor type: 300 MHz ; Min RAM size: 64 MB ; Min processor speed: 300 MHz

Spybot Search and Destroy 1.4 is a free, fast, proactive antispyware app, and it doesn't require you to provide an e-mail address or any other information about yourself.

Everytime I turn on the Windows firewall I receive a notice that Norton has a firewall and it recommends that I turn off the Windows firewall and use the Norton firewall instead. Secondary discussions about the inclusion of anti-virus and anti-spyware applications in the upcoming Windows Vista have decided in a recent poll which software they like most.

As per the result I have seen on the internet there are people who give most preference to Spybot, but I can just say that in my experience is best to use them both and use some other also. Adaware is still good as free version, you don't need to buy pro. A highly configurable advanced-mode interface is available for power users who like to schedule their scans.

Spybot has an advanced mode. In there you can find an uninstaller and you can optimize your start up. Not to mention spybots very large database of spyware and information on specific types of spyware with monthly database
updates.
TEKMATE