Both boards have the same microcontroller functionality and differ only in the cellular modem that’s built-in to the board. The MKR NB 1500 supports cellular network connectivity on NB-IoT and Cat-M1 networks, while the MKR GSM 1400 supports 2G and 3G networks.
While Cat-M1 networks represent the future of cellular IoT network connectivity, coverage is not yet available in all countries. Before purchasing the LTE-M Starter Kit (which includes the Arduino MKR NB 1500), check that Cat-M1 connectivity is available in your area. A full list broken down by country is below.
If Cat-M1 connectivity is not available where you are, you can purchase the 2G/3G Starter Kit (which includes the Arduino MKR GSM 1400), providing you the same prototyping functionality while connecting to 2G and 3G networks that are more widely available.