There are at least two key differences between drugs@FDA and openFDA.
A key difference relates to how the data in each is designed to be accessed. With OpenFDA, the primary way to access the data is by using the provided APIs. Drugs@FDA, on the other hand, is an online application that one can use to search for information on FDA approved drugs. Both openFDA and drugs@FDA, however, also provide downloadable files that can be imported into one's one tool or database.
Another key difference relates to the type of data each site provides. While there is considerable overlap (especially with respect to drug product labeling data), there are also major differences. openFDA currently has over 13 different datasets/APIs covering drugs, devices, and food. Drugs@FDA provides data on drugs. There is some information in drugs@FDA that is not in openFDA. For example, if you want to search for a generic drug product for an innovator drug product, then you would need to go to drugs@FDA.
To get a more comprehensive sense of the differences, you may want to take a look at the human drug datasets currently provided via openFDA datasets (see https://open.fda.gov/drug/) and also take a look at the FAQS page for drugs@FDA (see https://www.fda.gov/Drugs/InformationOnDrugs/ucm075234.htm#purpose).