http://compnetworking.about.com/od/dslvsca...l/aa111200a.htm
thats a good source for info.
my 2 cents. dsl runs on phonelines which are used also for voice therefore tend to be more oversold than cable where you only get video to each house. video runs in different frequencies on cable than data on dsl is the same however with voice over ip you are basically using the same bandwidth.
very easy laymans
dsl oversold more than cable
cable has its disadvantages too depending on the medium they use to transfer data from optic fiber to copper wire tends to have loss however overselling that is a bit lower.
take all this in to consideration and add 1 more variable upload and download speed.
if they are giving you asynchronous (which is a new trend in cable modem) where they give you more download than upload that means there is a pretty good chance they are overselling too. not sure about DSL however ADSL and DSL are basically the same the difference is that ADSL is giving you different upload and download capacities.
in my experience they both suck at least for office work. for house work cable is good. like it more than dsl.