The PCH component is essentially the system's chipset. It is attached to and cooled by the same heatsink as the CPU. As a result - and because it generates far less heat - it is not, IMHO, a good choice for the secondary temperature.
If you have an M.2 NVMe/SATA SSD installed, it will be the second largest heat generator in the system. The Motherboard sensor is located on the motherboard below where the SSD is installed. It measures the temperature of the air flowing between the motherboard and the M.2 SSD. If the SSD is hot and the air is not flowing well enough to remove this heat, the readings will rise to indicate so. Speeding up the blower will increase this airflow. Bottom line, IMHO, this is the best sensor to choose as the secondary temperature for fan speed control.
The Memory sensor is similarly located on the motherboard below where the SODIMM(s) are installed. Since the heat generated by the SODIMMs is far less than that of the SSD, IMHO, it is not as good as the Motherboard sensor at indicating issues with airflow.
Hope this covers it,