MLA International Bibliography is the research tool for literature because it most accurately reflects the state of literary studies in the United States and internationally. MLA includes listings of scholarly journal articles, books, book chapters, and dissertations, published from 1926 to the present. Abstracts are oftentimes not included, so you need to depend on subject headings to determine what the material is covering. Those subject headings are the key to organizing your search strategy.
MLA is excellent when you want to learn current research trends for an author or text.
JSTOR contains full-text articles published in scholarly journals from a wide range of disciplines, including language & literature. The most recently published articles (past 3-5 years) are not available, but much older ones are. Keyword searching can be tricky since JSTOR scans the entire article for your term(s). Choosing and combining your keywords is key.
JSTOR is excellent when you need historical research on an author or text, as well as when you want to learn who else cited a particular article.