An open loop pwm is not worth doing since the performance is so much lower than a "real" chopper. Unless the pwm freq is very high (hundreds of kilohertz?) it might not even work since the pwm will turn off the drive before the rotor have a chance to step into position.
Simplest way is to use a pair of comparators to sense the currents. If there's a PIC with 2 comparators, you won't need extra chips. Chopping is very demanding on the PIC. You only have 20-50uS to do everything. Adding an external monostable would make things much easier.