Some DSL modems offer a Universal Serial Bus (USB) port and an Ethernet port, so the user must choose which one to connect through. Ethernet is usually the right decision.
Prior to USB 2.0, speed was an argument in favor of Ethernet. USB 1.1 has a theoretical speed of 12Mbps but an individual USB device can only use up to 6Mbps. The problem with using a USB 1.1 port is that ADSL2+ streams data at a theoretical speed of 24Mbps. DSL modems with USB 2.0 ports instead of USB 1.1 ports are 40 times faster. They are not at a speed disadvantage compared to Ethernet considering that modem speed is limited by the Internet Service Provider more than the customer premises equipment. So, unless the modem has a USB 1.1 port, there are more important reasons than speed to prefer an Ethernet connection.
A better argument than speed is that USB ports are valuable for connecting many types of devices and should not be wasted on the internet connection. If the modem is attached to a USB port, that port (and its associated tax on CPU cycles) is not available for devices such as flash drives, mice, cameras and CD burners.
The best argument is that Ethernet exists and is optimized for shared communications networking. An internet connection via USB port is going to suffer from more dropped connections than an Ethernet link. Ethernet cables have a longer reach, easily 300 feet, while USB cables reach less than 20 feet. Ethernet does not require installation of a device driver, whereas USB requires device driver software, which adds complexity to the installation and becomes a source for potential problems. If you are on a Windows machine and want to switch to a Mac or Linux machine, the same modem will work with all three via Ethernet. If you are using the USB connection, the device drivers are for Windows, so you will not be able to use the USB connection if you switch away from Windows.
Ethernet provides the maximum sharing potential. By adding a broadband or wireless router, other computers, gaming consoles and devices can share the internet connection if the modem connects via Ethernet. Furthermore, by using the Ethernet port, it is possible to add a hardware firewall between the modem and the network or to use one of the DSL modems that contain an integrated firewall or support VPN passthrough (configurations not possible with a USB connection).
If your computer does not have an Ethernet port and you don’t want to pay for one and you don’t anticipate the need to share the internet connection, then DSL connection via USB is probably acceptable. For every other conceivable situation, connect DSL via Ethernet.