It depends on your style of studying & understanding. If you understand well through flow charts and case studies, then Kaplan is the right choice for you. Alternatively, if you study well by reading paragraphs then you should go for BPP.
Personally, i would recommend Kaplan. It is simple, well summarised and the the language is easy to understand. BPP contains lots of details, which creates jargon, and it gets difficult for students to remember most of them.