Having worked in Telco, there is both a technology and performance difference between DSL and Cable Modems or Leased Lines; DSL runs on a copper pair, which are prone to physical plant issues(signal loss from a variety of causes) but, often more importantly, run through a device known as a concentrator, which [which not surprisingly] concentrates traffic within a a network segment before handing off upstream. DSL capacity within that segment is constrained by the concentrator, so DSL is infamous for under-performing during work hours, and will often make VOIP un-usable.
As Indy pointed out, most consumer ISPs suffer from reduced bandwith during peak hours. It doesn't matter if you have cable or DSL. The way around it would be to buy your own T1 line (quite expensive at 300-400/month). But you get an SLA and guaranteed bandwith 1.5 Megabit UP and Down. Basically how consumer ISPs work is they buy the bandwith in bulk and sell it to the buyers. They might buy a T1 line and put 50 people on it or something like that for a very rough example.
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