Cable tends to be better. Phone lines in most places were laid with no thought for computer style connectivity. Phone companies tend to sell you their DSL services and hope for the best. If the connection is bad and enough people on your loop complain, they might fix it by upgrading the wiring. Or not. Basically, if you do get DSL because it's cheaper or it's your only choice or something like that, and it works right away, fine. If the company can't get it running smoothly within a very short time after installation, chances are it'll never ever work right, and you should ditch it right away while you're still in the money back guarantee period.
I'm speaking from research, personal experience, and what I' ve learned from guys I know at Verizon. Of course, different areas have different providers that may be more or less honest, but DSL providers mostly share the same awful wiring challenges. Good service people at any DSL company will admit this; I've spoken with some high ups at some DSL providers, and they've confirmed that if it doesn't work well at the install, their strength is not trouble shooting -- the unspoken message is that they're really more interested in volume at this point.
xoxo
Wyst