Neither. They both have a fundamental problem - there is no need to 100% finish USMLE World! Review is really important too. Here's what I would do (and I only studied 3 weeks, BTW):
1. Do as much of USMLE World as you can reasonably do. 1-2 blocks a day, review the answers thoroughly (even the ones you got wrong). Figure 1 hour for each block of questions and 1 hour for review. I did about 50% of USMLE World, and that was plenty of practice.
2. Get through First Aid. You can do this in 3 weeks, very reasonably.
3. BRS Path - definitely! If you like this approach, you can read each section directly after you go over the corresponding section in FA. Then look back at FA to see if there are any spots you need to go over in BRS again. I spent 1-2 days per subject/system in my 3 week schedule.
I also added BRS Phys - it corresponded nicely with FA and BRS Path and was a pretty quick read.
But, lastly, since I don't know your background at all, I have no idea what you need to do to pass. I think what I listed above will give you a good comprehensive review and should be fine if you just need to brush up on things, but 3 weeks does not give you much time if you are truly weak in any subjects and need intensive review. It's really important to objectively evaluate your strengths and weaknesses (not just what subjects/systems you like or dislike) and budget your time accordingly.
