Shin Kanzen >>>>> Sou Matome. It’s sad, but true.
Both books have their strenghts, tho. I used Shin Kanzen for N4 and just today finished Sou Matome N3 Grammar (I have the full set of Shin Kanzen N3 too, will use those after).
Sou Matome has the advantage of pushing for VERY quick learning while giving you translations, but the explanations are weak and at times nonexistent. Also, while the quick pace is great, I don’t think anyone will ever pass N3 after just doing that book in the 6 weeks you’re meant to use it. It’s too fast, has too little (read: no) reviewing, and the explanations are weak.
I do like it, but I got it knowing all of this. I got it as a crash course on N3 grammar before diving deep with the reading comprehension/listening practice books and the Shin Kanzen ones. It did its job, but nobody should ever take the Sou Matome set as their main study set. I had to actually refer to the Shin Kanzen N3 grammar book several times to understand grammar points Sou matome refused to explain.
As for Shin Kanzen, well, it has its own pitfalls - mostly, you don’t get translations for anything other than the explanations. This makes some grammar points difficult to grasp - the passives in N4, for example, become a horrible challenge since the use of passive form in Japanese isn’t the same as in English, by which I mean Japanese has some passives that don’t exist in English at all. However, the explanations the book has are mostly complete, and I can only recall having had to refer to internet sources a few times while doing the N4 book. It offers many more exercises than sou-matome, takes much longer time, and seems more intent on getting you to learn properly.
Therefore, I recommend SKM over SM any day of the week, having used both books. If you can get and use both sets that’s great, but if you can only choose one don’t go for Sou-matome. It’s not good enough.
I’ll add, however, that I believe using a proper (non-JLPT) textbook to be the best idea. It’s quite difficult to remember the uses of each grammar point if you never get to use it in writing or think about how to say this or that,and that’s something non-JLPT books will give you. JLPT books only try to get you to figure out what the right answer from a lot is, while other text books (Minna or Genki the most used ones) will get you to write, and think, using each grammar point. Me, I do both Minna No Nihongo 2 and JLPT practice at the same time. I’m far behind on MInna, but it helps me still by making me review older N4 points and helping me learn it properly.