Pardon my ignorance - but what is a PIM modulator ?

" need to know if its better to use interrupts or just one JB instruction waiting for tha pulse"

Use Interrupts if you want the MCU to do something else when waiting for the pulse.

Use JB if the MCU has nothing else to do otherwise. And just make sure that the particular pin used as input is prepared for it - else the wait could be eternal.

Have fun

Raghu