Properly executed, the direct drive designs out-perform the belt-drive designs, and dual-capstan designs out-perform single-capstan designs.
Most belt drive cassette decks use a simple DC motor, from makers such as Mabuchi, Sankyo and Matsushita. In time, these will fail, due to brush, commutator and bearing deterioration. Direct-drive motors use no brushes or commutators and use the long-lived capstan bearings as the motor bearings.
An interesting feature of dual-capstan designs is that the leading capstan is designed to have an ever-so-slightly greater angular velocity than the trailing capstan. This results in a constant, steady tape-to-tape-head pressure -- which leads to LOW dropouts. Dual-capstan systems nearly always use a belt to link the two flywheels together, and the condition and nature of the belt is extremely critical.
Fred Longworth