In the current and future versions of PBP, the PM assembler has been discontinued.
So it's probably best to write your code using the MPASM options, for future compatibility.

The PM assembler only worked with standard 12 and 14-bit core devices (12F,16F).
So for enhanced 14-bit cores (12F1,16F1) or 18F's, you had to switch to MPASM anyhow.
And since MPASM handles all of the chips, there's really no reason to be switching back and forth between assemblers.