What is the difference between the MKR WAN 1300 and the 1310? (I can't find anything on the Arduino Docs website.)
There is some information on the MKR WAN 1310 product page:
Better and More Efficient
The MKR WAN 1310, brings in a series of improvements when compared to its predecessor, the MKR WAN 1300. While still based on the Microchip® SAMD21 low power processor, the Murata CMWX1ZZABZ LoRa® module, and the MKR family’s characteristic crypto chip (the ECC508), the MKR WAN 1310 includes a new battery charger, a 2MByte SPI Flash, and improved control of the board’s power consumption.
Improved Battery Power
The latest modifications have considerably improved the battery life on the MKR WAN 1310. When properly configured, the power consumption is now as low as 104uA! It is also possible to use the USB port to supply power (5V) to the board; run the board with or without batteries - the choice is yours.
Data logging and other OTA (Over The Air) functions are now possible since the inclusion of the on board 2MByte Flash. This new exciting feature will let you transfer configuration files from the infrastructure onto the board, create your own scripting commands, or simply store data locally to send it whenever the connectivity is best. Whilst the MKR WAN 1310’s crypto chip adds further security by storing credentials & certificates in the embedded secure element.
So the MKR WAN 1310 is essentially an updated replacement of the 1300 and you should consider the 1300 to be an outdated product.
I believe the fundamental reason for the replacement is that the hardware design of the 1300 resulted in an unnecessarily high power consumption:
All else the same, if you have the choice between the two I would get the 1310. However, if the 1300 is all that is available or available at a significant discount and your application does not require a low power capability then I think it will work fine for you.