San Jose, CA
The above two posts, "DSL's whole problem" and "Re: DSL's whole problem", discussing the pointed problems with DSL and Cable, pretty much sum up the basic fundamental detriments of both services, technically as implemented.
It's worth saying that each one also has examples of where those technology deficiencies are not a problem, for instance, with DSL, being close enough and/or good wire ("cable" but that has multiple definitions, i.e., twisted pair, aka traditionally for telephone), and with "Cable" (i.e. coax, aka traditionally for television), nodes that are speedy (not overloaded).
There can also be some other differences: DSL is more often less restrictive in its acceptable use policies, and "cable" is usually faster than DSL. Depending upon the details of the available packages, your available budget, and what needs you have, those other differences are often important, and often are not.
What I advocate is seeing them as shades of the same thing, and learning what particular thing you have available in your home or business, so that you can make your decisions. A lot of people end up getting both, one at one time, the other at a time later, and go back and forth and finally settle on what works better for them.
The worst situation is when people are locked into contracts that they can't get out of to get something more appropriate for them. Locally, AT&T pushes terms onto resellers that require contracts, but they've gotten looser. Comcast doesn't do those term contracts. Also, Comcast costs more at the low end. Where I live, only one is fast; the other is slow. (You can guess which is which.) I think the contract lock-ins are worse than slow speeds, because ultimately they stifle competition and therefore quality, and therefore speed, in the general, bigger, long run.