Hi,
I was at a conference last year and asked this question in several occasions (around peers, in breakout sessions) but never got a straightforward answer. I did get a few points:
1. PBF was developed specifically for public sector
2. BPC was initially developed for private sectors
3. PBF is more geared toward IT people while BPC is more user frendly
4. For the reason of point 3., SAP came up with BPC RDS, in which there is a preconfigured public sector version.
One thing I do know is PBF has extensive form handling capability. In our organization, budget preparation does involve many, many form processes.
I would not feel comfortable to propose any solution to the business if I have not had any hands-on experiences on the pertantial solutions. I would definitly first ask SAP to do a demo for the customer. You will be surprised how many intelligent questions your customer will ask SAP during the demo.
Regards,
Ming