Scholarly Web Searching: Google Scholar and Scirus

Greg Notess puts them head to head...

Despite all the limitations and problems, both offer some unique reasons to use them beyond just watching their future development. For a quick, broad, multidisciplinary search on a very narrow, specific topic, either Scholar or Scirus can give a good start. For citation verification, both can help find erroneous as well as correct citation information. The Cited By links at Google Scholar can be a useful adjunct to the more comprehensive citation tracking from citation indexes via ISI's Web of Science (or can function as a partial replacement for those without access).