The good thing of Microsoft Antispyware is that it run scan all time and it can scan computer all the time. If there's any suspected software, it will ask you.

However, Microsoft antispyware may sometimes detect a program as spyware. One of the example is opera's banner. (Now no more since opera remove it's banner and official become freeware).

Ad-aware, in the other hand, is efficient and fast, but sometimes it may bypass some spyware compared to microsoft anti-spyware and spybot search and destroy. Overally I think that Ad ware is still the best.