What are they?
Memrise is an online memorization tool presented in a gamified way, with a leaderboard and badges. It’s set like a garden. You plant seeds (learn new words) and water the plants (review them). It uses flashcards, mnemonics and repetition. Anyone can create a course and it isn’t only used for language learning. There are courses on vocabulary for the SATs or the GRE, and even on Geography, to help memorize the world capitals or the states of the USA.
Duolingo is exclusively for language learning. Much like Memrise, it is also presented as a game, with a leaderboard, badges and unlocking levels. It can be used to practice both vocabulary and grammar. You can take a level test or start the language track from the beginning until you complete it. There is also an immersion section where you can practice translating real documents from the language you are learning into your own language.
How are they similar/different?
Both are similar in the gamification aspect and in that they incorporate both audio and text in the input. The activities are also similar. There are activities in which you have to write what you just heard, others in which you have to put the words in order to form a sentence, and others in which you have to select the right option.
They are different in their approach. Memrise is a memorization tool while Duolingo’s approach is more complete. I would use Memrise to memorize vocabulary and Duolingo for the grammar. You should be warned, though, that Duolingo’s sentences can be quite random. I have used both to practice Portuguese and one of the sentences that I got is “a borboleta escreve um livro” (the butterfly writes a book). But then again, maybe such random sentences help with memorization. Case in point, I still remember it.
Personally, I find both useful if you are starting with a language or want to practice it in order not to forget it but what I find lacking is the chance to actually speak the language. If you can combine them with some kind of language exchange or conversation table, then they are great tools which are also fun and motivating. It's hard not to practice a language when you only need a few minutes using an app in your phone and you get a notification reminding you to water the plants (in Memrise) or telling you that you are in a three/four/whatever-day streak.
Have you tried Duolingo or Memrise? How was your experience? Did you find them useful. Let me know in the comments. Thank you for reading :)