Here is a very good FAQ:
Q: DSL vs CABLE? (#129)
A: This is a question that is asked everywhere you look. Which do I want ... DSL or Cable?
DSL service shares bandwidth amongst ALL users connected to the same DSLAM. Cable shares bandwidth amongst ALL users connected to the same CMTS.
DSL's advantage?
The dedicated circuit prevents other users from affecting your connection to any significant degree. (In most cases.)
Cable's advantage?
Generally cable can support higher bandwidth rates, and can usually provide service to a larger area than 18,000 wire-feet, DSL's limit.
Cable modems are typically faster for downloads than most if not all DSL lines, when the cable infrastructure is new or well maintained. One of the most common complaints seen in our cable forums is that of increased latency and other problems as more subscribers in a given area come on line. Additionally, cable has a few other disadvantages when compared to DSL.
The first disadvantage is that cable is an RF network -- this means that it is vulnerable to transient problems "within the network" from RF interference. Since cable is a shared media, there is a possibility that performance may degrade over time as additional households plug in, connect additional devices (videos, game machines etc.) to the TV lines.
A cable company may react slowly to decreases in performance if it reacts at all, as they never sell access by speed, or promise consistent speed or latency.
Another of the disadvantages of cable over DSL is the upstream (return path). Cable companies are using a very narrow band for return signalling, and this is positioned below all the space allocated for TV channels. This band is prone to RF interference and is very limited in capacity. Upstream transmissions may therefore compete with others in the area, get delayed (suffer high latency) due to noise fighting techniques, and cable TOS (Terms Of Service) typically prohibit any kind of constant upstream use. Internet use is shifting away from central servers broadcasting to many individuals and some interesting peer to peer applications are appearing (games, voice and video applications, communal libraries). These applications need a strong upstream channel.
In summary, cable modems are currently good value and strong competition for residential casual use, often available more cheaply and far faster than their ADSL competition. However, DSL is probably the more future-proof system, offering digital direct from the internet infrastructure. If your DSL ISP is on the ball, your performance in either direction will not be different from peak hour to early morning, and DSL lines are available for a wide variety of purposes, both business and residential.