"Phil Simms is the greatest QB in Giants history" ........Mahatma Gandhi
They evaluate different things. FO is purely based on the NFL gamebooks which track things like down and distance, where the run went, and that sort of thing.
PFF is based purely on film watch, and they can track things like hurries, which the NFL gamebook does not record.
In terms of pass blocking, because PFF tracks more data relevant to it, then PFF would be more accurate.
But none of that is even relevant to run blocking. Which you can use the two to compare and find an average if you want. Not overly surprising, but the stats based and film based ratings are often very similar.