The main difference is the maximum speed. You will need a DOCSIS 3.1
modem to obtain over 1 Gigabit/sec data rate. Some DOCSIS 3.1 modems
have two ethernet ports that can deliver 1 Gigabit/sec each. Others
have a single 2.5 Gigabit/sec ethernet port. Still others have one of
each. This might help if you're comparing features:
"DOCSIS 3.1 Modem vs 3.0 Speeds and Testing"
<https://us.hitrontech.com/learn/docsis-3-1-modem-vs-3-0-speeds-and-testing/>
Note that a DOCSIS 3.1 modem is downward compatible to DOCSIS 3.0
speeds. If you want to future proof your modem purchase, you can buy
a DOCSIS 3.1 modem and use it at DOCSIS 3.0 speeds until you decide
you need speeds over 1 Gigabit/sec.