Part of deciding where to search is recognizing the differences between tools. The search scopes of Google Scholar, Web of Science, and a disciplinary database are fairly different, but also have some overlap.
Google Scholar searches across resources from all disciplines/subject areas:
Web of Science searches citations of high impact journals in the field (not every journal in the field, so Web of Science is less comprehensive in regards to discipline).
A Disciplinary Database searches a defined set of resources, all focused in that field of research, usually this includes:
A disciplinary database identifies the citing articles within its scope (the journals it indexes). In this example, within the database PsycINFO, this article has been cited 223 times since 2001.
Web of Science found 122 citations to this article in its collection. These citations will be from the other high impact journals that the Web of Science tracks.
Google Scholar's scope is larger, therefore it can identify that an article has been cited more times across a larger body of literature or types of sources (such as books and reports). Due to the unmediated nature of Google Scholar, some of the "cited by" citations might also be duplicates. Google Scholar reports this article has been cited 344 times since 2001.