Memrise actually uses science and statistics to enhance your learning of whatever it is you are learning.
I used it a LOT at GCSE and a bit at AS, picking it up again now.
I had duolingo for a few weeks but was not seeing results with it and it seemed a bit simplified tbh
I'd say give both a try but be aware that it depends what you are learning and how you learn best.
Memrise was so great for me because I could set goals (minutes of it per day to achieve), the app is great, so is the online one and I can listen to it and repeatedly type out words (this is how I learn best).
You can also make visual aids for memorising certain things.
@Quizlet also gets a shout out because it is fab for making your own revision materials but using existing ones and learning a lot is best on Memrise.
My patience was low for duolingo so I'd suggest you try it but yeah.
Worked for me especially for languages and I also used it for GCSE definitions in Geography, Maths and Chemistry.
At AS I used bits for French and Maths and Chemistry definitions.
At A2 I mostly use Quizlet but am picking up a Chemistry and 2 language courses to bring back the fun into my week
Hope this helps, erm excuse the rambling but these things are sooo exciting