Typically, cable modem users tends to have faster download speed than adsl because they have more bandwidth per head-end.

There was a myth that cable modem are slower because of the network 'sharing' among the neighbourhood but today, in terms of actual usage, they're about the same because it all depends on the service provider oversubscription ratio.

For example:
Provider A, an ADSL provider, sells standard package for 1Mbps/512k upload.

Provider B, a cable modem provider, sells standard package for 3Mbps/256k upload.

On Day 1, both provider A and B, each has 500 users per head-end on its trunk port (say the trunk port is about 100Mbps). All else being equal, users on provider B can download faster than provider A.

As the day goes on, both provider A and B sign up more customers, and they continue to use the same single 100Mbps trunk port up to support up to 1000 users per trunk. By 30th day, say provider A has 700 users, and provider B has 900 users. Trunk port on provider A is at 80% utilization, and trunk port on provider B is at 95% utilization, users on provider B will experience slower access than provider A.

But, say on one typical day, the trunk port on provider B drops to 60% but trunk port at provider A remains high at 80% utilization. Now, users on provider B will have much faster access than users on provider A.

You all see now? It all depends on the providers, how they 'engineer' the bandwidth to end users.