NY Muscle is right in that if you get a cable modem and the company that runs the service doesn't upgrade the local routers, then they will effectively get clogged as too many users in the area are on for them to handle... the result is your network connection can get really slow.
a good company won't do that, depends on where you are.
DSL is slower at greater distances from the repeater, and it slows down in hot weather (it runs over copper cables, so you are dealing with thermoelectric principles). so if you live in the middle of nowhere, or in a very hot area, DSL isn't gonna be feasible. The colder it is, and the closer you are, the faster it will be by default (not the case with cable).
also, DSL is usually fast down, and slow up (meaning if someone wants to connect to your machine, it will be slow) - whereas cable is traditionally equal speed either way (changing some now as AT&T and others are creating variable speed plans to generate more money).
check out
www.dslreports.com - they cover all home networking, both DSL and cable - they have an extensive list of areas and a good discussion board for all kinds of things.