What's the difference between grains and PF - tek substrate?

Colonization Time:
The main difference between whole grain and PF style jars is that whole grain remains loose, and can be shaken, thus spreading the mycelium points and colonizing faster.
PF Jars are solid and cannot be shaken.

Sterilization:
The PF-tek substrate can be sufficiently sterilized even without a pressure cooker while whole grain virtually requires a pressure cooker to sterilize effectively. (see also "fractional sterilization")

Inoculation:
Grain can be inoculated with with a syringe, agar wedges of grain to grain transfer, while the PF-jars are limited to a syringe.

Bulk Spawn Use:

This bulk substrate is typically 1-10 times the volume of the spawn grain and significantly increases yield per given volume of spawn. PF cakes are primarily a fruiting substrate but can be used as spawn for a bulk substrate, however they are not as effective as grain spawn.