Theologiae is the winner. According to the case-sensitive Google’s Books Ngram Viewer, “Summa theologiae” overtook “Summa theologica” in the mid-1970s, and “Summa Theologiae” overtook “Summa Theologica” around 1990, at least in books written in American English. For books in British English, the respective changes took place around 1980 and the mid-1980s.