I took both classes with the same instructor and left with the impression that
A.) CCPs were to be focused on the "truly critical" (her words) steps to prevent, eliminate, or reduce. PC's were broader and focused more on proactive preventive measures in areas that didn't always fit in the historic definition of CCPs.
B.) In cases where you have to have both HACCP & HARPC plans, your CCP would also be a preventive control, but not automatically the other way around.
To me, the classic cook an egg to X temp for Y time is a good example of a step that would be both a CCP and a PC.
A PC alone might be proactively monitoring your quick cool processes to avoid possible microbiological growth because current science says you have to cool your product within X time to a Y temp or face a 1/2 log increase in the microbial load. It may not be "truly critical" (1/2 a log) and therefore you might choose to not define it as a CCP, but is something that should be overseen/monitored since the science says that you would be adding to the load and none of us should allow that to happen if we can prevent it. By making it a PC, you would be officially aware of it and implement steps to assess contamination/risk/corrective actions, etc should product chill times go outside the established limits.
I honestly think that much of the confusion is because people were already doing preventive measures (think the quick chill monitoring above). If you are good at your job, you are looking for possible issues and seeking to prevent them. HARPC just formalizes that process, names them something new, and possibly adds to the monitoring/verification involved.
How many people had extra steps in place years ago to try and prevent allergen cross-contamination? Well now you would formally include them into your HARPC plan and rename them as sanitation and/or allergen controls. Same results mostly likely, new names and (more) documentation.
My biggest complaint with all of this is that it just adds (more) complexity where reducing complexity improves the outcome.
I now have four separate analyses (HACCP, HARPC, Defense & Fraud). Waiting on the one to cover aliens (or would that fit under food defense? Hmmm.... I wonder.)
Not sure if I've helped anyone or just confused myself further.... 
Good luck,
Todd