Modbus:

• All communication is initiated by a master/client device.
• The slave/server devices send a response back to the master/client, confirming the request or indicating an error.
• Modbus primarily works with coils and discrete inputs (boolean values), and holding registers and input registers (16-bit integers).
• In Modbus TCP, multiple master/client devices are permitted. (This is not the case with Modbus RTU.)
• Modbus tends to have good support with industrial devices.
  • The AutomationDirect CLICK series of PLCs are built around Modbus.
  • Siemens PLCs are able to communicate using Modbus TCP without a hardware adapter.
  • Universal Robots (UR) can be setup as a Modbus TCP server.

MQTT:

• Requires a broker device.
• Messages have a topic and a payload.
• As far as I know, the payload can be any sort of data.
• Any device can publish to the broker.
• The broker will forward the message to every device subscribed to its topic.
• Works extremely well with Node-RED.