entireley on where you are located. DSL is transmitted over a phone line. There must be a phone line. Whether the phone line is also used for phone service is technically irrelevant, but absolutely relevant to the business practices of the phone company that owns the line leading into your house. Without information about who they are, we can't tell you much. Verizon has been mumbling about allowing naked DSL (that's the jargon for DSL over a phone line with no dialtone/voice service) but nobody has seen it yet. Qwest has offered it in places. Other big telecos may or may not, it's all pretty ambiguous - but as I started to say, it depends absolutely on the business intentions of the one phone company that owns the one line into YOUR house. Anything going on anywhere else means nothing.
Note that there may be other ways to get DSL - e.g., resellers such as Covad - but they can only help you if they have forged a business relationship with the aforementioned one phone company that owns the one line into your house. Sometimes, the reseller can lease the currently unused phone line into your house and then transmit their own DSL over it without simultaneously providing a dialtone. But again, that all depends on the business intentions of the phone company whether they'll allow a competitor easy access to the circuits. Technically, they are supposed to. In reality, the phone companies treat the outsiders miserably, which often trickles down to miserable service for the customers of the outside company.
As for the blanket statement that cable is better than DSL - that too depends entirely on what is going on in YOUR neighborhood. Some cable companies suck big time. Some are outstanding. Some DSL plans suck big time. Some are outstanding. Without knowing the exact details of YOUR options, we can't make a recommendation.
dw