Cable modems can theoretically offer greater download speeds.
But the problem with this option is that bandwidth is shared among all cable internet users in a neighborhood, this affects your download performance as more users in your neighborhood log in to the network at the same time - Picture a garden hose with one small leak in it. Then picture the same garden hose with hundreds or thousands of leaks. It's the same principle. Upload speeds over cable modems can in many cases be slower than DSL, as well. Usually it's because too many people in a neighborhood are trying to send or receive data at the same time.