Both are good resources for different reasons.
The Kaplan BS book has excellent descriptions and examples that KA does not necessarily provide in such a concise manner. That being said, KA has a greater breadth that it will expose you to. With the P/S section, your baseline has to be knowing all of the terms and concepts covered in those resources. However, unlike previous iterations of the MCAT circa 2015-2017, the exam has shifted away from being about memorization to being about reasoning. IMO, the P/S section is similar to CARS but with a heavy content focus. Given the high raw scores in the P/S section, to earn a top score you really have to be on your game. An additional 1 or 2 questions correct can mean the difference between an additional point in your scaled score.
My advice would be to use both. The Kaplan book is pretty short. I used to read one chapter a day before bed and ended up reading the book about 4 times. I used to read the KA text during study sessions and read through it several times.