Duolingo to be a fully serviced learning experience for foreign languages.
The primary difference between the sites for language learning is that Duolingo offers pre-written courses, where you can learn a language with a full guided journey throughout their syllabus.
Memory is a very different type of system, where typically users will create their own courses with their own terms for self-study.
Whilst Memory does have pre-built courses available, Memory is specifically designed for those who wish to learn specific words or vocabulary - by way of adding the terms you wish to learn, and using our system to practice and learn them.
Many users find Memory to be useful as an adjunct to their traditional language learning - or for those who wish to study specific vocabulary and topics to pass or cram for exams.