If a fixed data rate is required, then SF10 would be better; hardcoding SF11 and SF12 is not allowed on a public LoRaWAN network, and you might get you a message from TTN on day. 
If a fixed data rate is required, then SF10 would be better; hardcoding SF11 and SF12 is not allowed on a public LoRaWAN network, and you might get you a message from TTN on day.