Kaplan, Princenton review etc all say they are the best TP any data to know where to put my $$?

All test prep keep saying they are the best prep so buy them. Anecdotally, they all say our students got the highest scores. We are all scientists is there any science or study or data collected as to how students did with different test prep groups? And where students did best with each TP? I don't mind studying hard but stuck not knowing yet where to spend my hard-earned money. Everyone says their students had the highest scores and in some cases, my colleagues bought into it and wasted... Kaplan, Princenton review etc all say they are the best TP any data to know where to put my $$?
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums