SKY is correct about cable. Cable is configured in a star configuration, so when more people are added to the star your connection can slow down dramatically (unless they agree not to add people to your star). DSL is single line, or home run configuration that should not vary in speed once it is set up.
That's interesting. In the U.S. (at least where I was at) DSL was horrible. It was always slow, much slower than it was "supposed" to be. Cable was always dead on. I was told that the further you were from the source the slower DSL would be and I lived in a rural area so I figured that was right. But networking stuff is not my specialty so I'll defer to those who know more. Does that argument hold any water in your opinion?