The Raytac USB Dongle is recommended for the majority of users.
Users who wish to implement advanced firmware integrations that require access to GPIO pins and/or Serial Wire Debug may prefer the Nordic nRF52840 Dongle (this product). If you are not sure what those are, no worries! Most like you would want the Raytac dongle.
Please note that no support is provided for custom integrations that you may choose to implement.
This dongle comes preloaded with the Adafruit UF2 bootloader just like the Raytac dongle so it to be used with the keymap configurator out of the box.