I would say TBR was a little harder. At first, I missed like 2-3 questions on TPR passages, and I was scared ****less. As I practiced more, I started to read the details closely, think about the concepts before answering the questions...and right now, I have only missed 4 questions out of 20 passages. WIN!
TBR fluctuated for me. There were sections where I only missed one to two questions out of 7 passages. Other sections, I missed like 1 per passage.
My advice would be to NAIL THE CONCEPTS BEFORE DOING PASSAGES. This helped me a great deal. I know the content in Physics and Chemistry inside out, and I can easily connect the dots...and the results show my mastery.
I would say do all half of the TBR passages, then the corresponding TPR passages. The last 1/3rd of the passages in TBR...save them for last month testing while you take your FLs.