PBL is implemented differently at different schools... but I think that in general PBL is more interactive and student-directed than a SBL curriculum.
As a participant in PBL, I feel like the curriculum is pushing me to build personal interaction and information processing skills that an SBL curriculum wouldn't. I also feel like I won't necessarily be exposed to all of the medical knowledge that I need to prepare for boards in class-- that's ok because I feel comfortable learning on my own.
That's not too say that a student in an SBL curriculum doesn't develop these types of skills, I just think that the SBL curriculum focuses on exposing students to quantities of medical knowledge rather than developing problem solving skills.
The question might be what do you want your medical education to do for you and what do you want to be responsible for?