i did both last year, and i'd say they both have their challanging aspects.
i found the IRP to be less stressful than the PIP, obviously the PIP is externally marked so you can't gauge on how the marker is going to react to your topic.
the amount of detail in every part of the syllubus in cafs was amazing - there's so many branches in the subject it could sometimes get overwhelming.
sac was more globally based, and had concepts that related to every topic you did, so that made it easier to link things together, although the content was so expansive so it sometimes became a bit unmanageable.
well, that's basically what i found anyway