Originally Posted by Wheezer
Cable will indeed fluctuate based upon local demand, since users share the distribution system. With DSL, it's just the copper from your phone jack to the DSLAM.
Which is where DSL users "share the distribution system", resulting in the same "fluctuations". Engineered correctly, either will get you the speed committed to. Chose based on price and service/support.
As a datapoint, my 3Mbps cable regularly measures at 3.7-3.9 any hour/any day.