Mobile Internet does not use Telkom's DSL infrastrcuture and can therefore be used anywhere where there is MTN coverage and traffic is routed directly onto MTN's fibre optic network.
ADSL used existing Telkom copper/fibre infrastrcuture and is router via DSLAM exchanges to the closest IPC on the MTN network and thereafter onto the MTN network to route locally or internationally.
Both products offer great value depending on your requirements. DSL offers a lower per GB price and generaly is the best option for low latency gaming or streaming. Mobile Data offers much lower overheads (in terms of monthly rental) but is more expensive per GB. It has the added bonus of being available nationally, so you have data pretty much wherever you go.
For most people, who want to be connected 24/7, it's mostly important (and affordable) to have a combination of both.