When should I choose to use a CAT M module over a CAT 1 module?
Both use the same LTE network. The main difference is that CAT M is designed for devices that communicate sporadically or occasionally; thus the XBee cellular module may be in a low power sleep state most of the day. The data rates on the CAT M are up to 350Kbps for downlink and uplink.
The CAT 1 is designed for devices that are not concerned as much with power consumption and wish to send data frequently. The data rates for the Digi XBee3 CAT 1 are 10 Mbps downlink and 5 Mbps uplink.