This paper reviews a number of studies comparing Thomson Scientific's Web of Science (WoS) and Elsevier's Scopus. It collates their journal coverage in an important medical subfield: oncology. It is found that all WoS-covered oncological journals (n=126) are indexed in Scopus, but that Scopus covers many more journals (an additional n=106). However, the latter group tends to have much lower impact factors than WoS covered journals. [...]
Results reflect fundamental differences in coverage policies: the WoS based on Eugene Garfield's concepts of covering a selective set of most frequently used (cited) journals; Scopus with broad coverage, more similar to large disciplinary literature databases. The paper also found that 'classical', WoS-based impact factors strongly correlate with a new, Scopus-based metric, SCImago Journal Rank (SJR), on a series of new indicators founded on earlier work by Pinski and Narin (1976).
(López-Illescas, C.; de Moya-Anegón, F. and Moed, H.F. Coverage and citation impact of oncological journals in the Web of Science and Scopus. Journal of Informetrics, n. 2, pp. 304-316, 2008).