I would say Renshuu is MUCH, MUCH, MUCH, better at learning Japanese!!! Before I knew about Renshuu I used Duolingo.
I didn't know anything about Japanese including the "alphabet". So I needed a basic understanding. But, instead of teaching you the symbols Duolingo goes straight to words which is a terrible strategy, I mean how are you supposed to know how to write the words if you don't even know what "letters" their made up of!! On top of that the accuracy isn't very good (especially since they use AI which is most likely based of "Google Translate")
I also found that Duolingo's pressure to keep your streak is really irritating ( and honestly does more harm then good ) unlike Renshuu who actually encourages breaks when you need them!! When I first started Renshuu I felt like I was actually learn things too, it was a different,an almost relaxing feeling when I first started out.
The customization in your learning really allows the users to decide what they learn and at the pace they prefer, unlike Duo who who gives you the same pre-made paths that you are stuck with your whole learning journey.
The community involvement is another bonus too. Renshuu encourages community engagement, whether its playing Shiritori, dissucusing with others in forums, entering Kao-coin contest, or even attending the discord meetings, this all helps the users feel encouraged to keep learning with others. But again last time I checked the only engagement with the community that Duolingo has is the leaderboard... ( which i'll be honest sometimes it feels like I'm competing against bots!)
I'd also like to point out another bonus which is the.... ZERO ads!!! like zero!! Renshuu doesn't force you or pressure you into getting the pro ver. it literally tells you that it is a 100% optional and on top of that gives a free trial (again, FREE) when you first start out!!! I mean how amazing is that ( finally a learning app that isn't greedy!!) Duolingo on the other hand pushes ads to you after every lesson to buy their Super/Max ver. (which I find funny because they say you won''t get any ads if you buy it but the only ads you get are the ads pressuring you to get one of the upgraded ver.)
I would also like to add a comment to the "energy" thing they have going on, you know, the thing you have that lets you complete a lesson. I mean what do you mean I LOSE energy when getting a question RIGHT, like are you really punishing me for learning!! I mean at least you could practice to get a heart (the old "energy") but still why limit the users learning like that, it's almost as if they don't want you to learn!! but it's ok because... Renshuu comes to save the day again!! Renshuu lets you review materials again and again( in the form of a "focused review" with no limit!!
Not to mention that they use AI to help you learn... I mean really, AI. I mean you might aswell just head on over to Google translate and ask it to teach you Japanese...
Overall Renshuu really is ( in my opinion) the best Japanese learning app there is out there and it makes me happy to see that more and more people are starting to realize the flaws that Duoling has, now that doesn't mean that Renshuu doesn't have it's flaws, but it's pretty darn impressive that all of this was accomplished by two people who wanted to help people accomplish their goals.
( I hoped this helped!! Also this probably has a lot of typos, but I hope my message still comes across, sorry for that!! )