In my opinion it’s like comparing apples to oranges. They’re both great, but they accomplish different goals. Duolingo is more of a general language learning site with a nice learning curve that everyone can enjoy, whereas Memrise is mostly vocabulary.
They work bast hand-in-hand in my experience. Both use different methods (gradual progression for Duolingo and spaced repetition for Memrise). But If I were to choose, I’d say Memrise is slightly better because Duolingo has a finite number of predetermined courses while Memrise/Decks has literally infinite possibilities (in terms of vocabulary) if you’re willing to allocate some time into making your own course.
So bottom line : Duo - great and very beginner-friendly. Memrise - a step beyond that.