Both are good. It depends on how you understand the concepts. If you understand well through flow charts and case studies then Kaplan is best. If you prefer studying paragraphs then use BPP. Personally, I'd suggest Kaplan as it is way simpler than BPP. BPP contains lots of details which creates jargon.
I'd say Kaplan. It is easier to digest the knowledge than BPP. I'm not sure if it is the layout of Kaplan's text but I understood Kaplan better than BPP.
BPP is better because it is neatly organised, to the point, and exam focused.
I firmly believe that Kaplan books are better than Bpp ones.
I use Kaplan textbook and BPP revision kit. Open tuition tutor doesn't recommend to use books at all. He claims lectures plus revision kit is a key to success.