I’ll preface this by saying that I’m not a shill for LingoDeer, hehe, this is just my experience as somebody who has used both platforms extensively.
Historically Duolingo been very poorly regarded for a number of reasons, not least machine generated audio and a very much “teach you stuff and not explain any of it” in a way that works for Latin languages, but not so much for Asian languages. They may well have made improvements to the course over over the years, I’ve not bothered to try progressing the tree, if I’m entirely honest, but it still starts off pretty poorly in my opinion by trying to do the same thing they do in every other language tree. They mix learning kana up with greetings, seasons, colours, etc. which is pretty weird. In that way, I’d rate it up there with Rosetta Stone.
I genuinely have nothing against Duolingo, I’ve had a plus sub for a while, and I used it for completing the Swedish tree which I enjoyed very much. However, the system was built from the ground up for languages that just have to deal with different words, not two syllabary sets and kanji. For Swedish, I didn’t need external grammar or vocab resources, for example.
The reason a lot of people recommend LingoDeer instead is that it was built from the ground up for Asian languages, features native speakers and has fairly extensive grammatical explanations at the start of each lesson. It has, in my opinion at least, a much better internal review system, allowing you to select sets of review items like you would with an Anki deck. It’s not SRS, but it’s perfectly fine for reviewing as and when you have time.
The course on LingoDeer also makes it clear how it equates to testing levels, the Japanese 1 course mapping to N5 content (so roughly Genki I) and the Japanese 2 course mapping to N5 (roughly Genki II). This is the range of material available, for example:
In my opinion the grammar is far better explained, in a more logical and textbook fashion, meaning that you don’t actually have to rely as much on external sources. Though obviously it’s better used with other material, it does technically cover N5 and N4 material, so you could (though might find it hard) use it as a comprehensive study resource. I personally find that with it, paired with BunPro and WK, I’ve got a reasonably rounded beginner level Japanese experience.
Here is an example of a LingoDeer lesson page vs DuoLingo:
I tried to find lessons that matched, more or less, though one is dealing with language and the otherwith nationality
Edit: I forgot to add, Duo also doesn’t seem to offer any kind of customisation, whereas LingoDeer will let you choose to display vocab as; Kanji, Hiragana, Romaji, Kanji + Hiragana, Kanji+ Romaji, Hiragana + Romaji, Kanji + Hiragama + Romaji.
They also let you choose either the Hepburn or Kunrei-sheik romaji system.
I did Spanish for a while and didn’t have anything like this.
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