To access APA PsycInfo, use this link:
(N.B.: These materials refer to the old interface for APA PsycInfo. They will be updated with links to resources for the new interface when those become available. For a general overview of the new interface, see this video.)
Basically, APA PsycArticles makes APA PsycInfo better. In almost all cases you should still use PsycInfo as your go-to database for social and behavioral sciences.
APA PsycInfo is an index that seeks to give a comprehensive overview of the scholarly literature in the social and behavioral sciences, but many of these articles are not included in full text. By adding APA PsycArticles, we now have full text access to American Psychological Association publications for which you could previously find brief entries in APA PsycInfo, but you would have had to find in print in the library or request by interlibrary loan in order to read the full article.
In general, unless you know for sure that you are only interested in results from official publications of the American Psychological Association, you'll get better results by searching in APA PsycInfo. APA PsycInfo will give results from American Counseling Association and other publishers as well as American Psychological Association; APA PsycArticles will only give results from APA publications.