Genki vs. JFZ
In general Genki and JFZ will get you to similar levels (about N4 if you go by JLPT) though they don’t cover exactly the same grammar points. Genki has 2 books written like typical textbooks, though the bulk is exercises with pair work (you can still do them alone if you use your imagination). JFZ is 5 books and written in a more relax way I suppose you could say. Genki also has workbooks to go with the text books. JFZ has workbook sections worked into the textbook portions. The first 2 books of JFZ are a lot slower paced than 3-5 as those volumes slowly work at teaching hiragana and katakana, kanji starts at vol 3.
As for which has better practice problems, it’s a bit subjective. They both have similar style of writing your own kind of answers. (Full disclosure, it has been a while since I’ve done the Genki ones) JFZ has a bit of reading comprehension in each workbook section whereas Genki has these in the back of the textbooks. JFZ has kanji (vol 3-5) in the chapter lessons, Genki has it in the back of the textbook and again in the workbook. So the flow of each is different. I don’t have the Genki workbooks anymore, but each chapter had a page in there or so for grammar and also something for kanji practice. Adding pair work and the workbook together Genki might have more practice work than JFZ, but whether or not it’s useful or valuable… is up to the individual to decide.
Speed
Which one would get you from point A to point B faster? Depends on how fast you are willing to go. JFZ has fewer long grammar explanations and no pair work and no flip flopping around the book compared to Genki. If you skip the pair stuff in Genki and the kanji that will cut some stuff out. Genki has reading sections in the back which some people may not go through. It’s really down to how fast you want to go through the material and still retain info. This applies to both series. JFZ vol 5 is much more dense than vol 2 so it will vary by volume there.
Additional Resources
Genki has an app with all the audio and I think other stuff…? I don’t use it so I don’t know, but it has that for free. JFZ has free YouTube videos that go along with each chapter (they are still being completed for the later books) and there is also a website you can use which is like the book only different that has the audio and also chapter quizzes and things. It is something you have to pay separately for though so if you buy the book you don’t get free access or anything (I think it’s free to start, but not to do everything, someone else posted the link above so you can check it out if you want). Lots of people do stuff with Genki online and on YouTube (like ToKini Andy’s YouTube off the top of my head), some are free, some are not.
My Opinion
I have gone through both Genki and Japanese From Zero in their entirety. Genki I went through in college with an awesome professor so it was a great experience because of that at the time. Ultimately, I didn’t keep my Genki books because they got ruined, but I chose not to buy them again because they were boring for me and I didn’t need them around. I went through the JFZ books because I wanted to know what the experience was like not because I needed them, though there were some things I picked up anyway. I don’t really get why they are considered just for kids (or better for kids/teens) because they are pretty general and there are some jokes in there that I think are aimed at adults, maybe because of the graphic style? My point is, adults can use them just fine. I actually prefer the vocab lists and the reading bits in the workbook sections of JFZ over Genki because it doesn’t assume I’m still in college on an exchange to Japan and want to talk about my major in school to everyone I meet. I also like that JFZ doesn’t have a set narrative. I get why Genki has does it as it’s designed specifically for college/university students, but with those days long behind me I find it tedious (and my major isn’t even on the list!).
Quality/Quantity
My opinion here is that when you are just starting and are brand new stick with one source that works for you for your main grammar text. If you want to use other things like BunPro as a study tool, or Wanikani for kanji, that’s fine because they don’t count (BunPro links to sources, but is just a study aid and WK is obviously not grammar). But multiple textbooks when you are just starting is unnecessary and likely will be more of a hinderance than a help. Having too many sources can easily overwhelm a new learner so add sources later and for specific reasons as needed. There is no need to be a textbook/dictionary/reference book hoarder like me (but I enjoy it, so there )
Those who have specific goals or timelines will have different needs, of course, and a textbook like Genki or JFZ, etc. may not be appropriate for all learners and all situations. It’s good to know what your needs/goals are before choosing something I think. If your goal is to pass the JLPT at the N1 level in a year, it is highly unlikely Genki would be useful to you in that time.
That’s all I can think of for now. Fair warning: I edit posts with wild abandon so…
Edit (I did give fair warning): An Integrated Approach to Intermediate Japanese would be more of a Genki 3 than Tobira as it is by the same publisher as Genki. However the structure is not like Genki and lots of people don’t seem to like it so Tobira is winning the Intermediate textbook battle at least on WK it seems (maybe elsewhere too, I don’t know). I owned it at one point but discarded it as it was super boring to me.