Which is worse: 
- You have a DHCP internet client that practically never changes its IP address. You also have a firewall that only updates a DDNS service when the IP address changes. The service will erase you if it does not get modified every 30 days (way more frequently than your DHCP IP address changes.) So you have to go to a website once a month and renew your existance.
- You have a DHCP internet client that practically never changes its IP address. You sign up with a free DNS service so that the IP address gets translated into a name. But if the IP address handed out to you changes, your DNS entry won't be updated until you go update it yourself manually.
The first option means you have to regularly go update something. But the second option means that you won't have to do anything until your IP address changes… at which point you will have lost or forgotten the magic to get in and change your DNS name.