From here shared bandwidth to the Internet is used as explained under cable modem discussion. Also, like the cable modem, XDSL connection is always on. To utilize this technology, the user installs a splitter at the premise, and an XDSL modem which connects to the computer. Regarding security issues, the industry is working very hard to counter security risks. As is true with any technology that always stays connected to the Internet, there is an element of risk.
Common sense approaches to guard against security situations, are usually helpful.
This is a good question with no simple answer. At this time, many users in large metropolitan areas have options for either technology. Many others have only one option or none available to them. In rural areas the telephone wires may be longer than the maximum limit for the XDSL technology. Thus, the present XDSL technology may not be available to these users.