One can argue that DSL is dedicated between you and the Central Office (and shared from there on), while Cable is shared for that "last mile" segment of the connection as well. However, cable technology is able to push much higher bandwidth over that last mile to support multiple clients, and the signal does not deteriorate nearly as fast because of distance. Because of this, cable modem technology may be somewhat more prone to variations in speed than DSL, however, it usually offers higher average throughput.
Note: One can compare a speed test from their ISP's servers (local/nearby test) vs. a speed test from a distant location to determine whether the speed limitation is in the last mile.