I am planning to buy a two wheeler and wanted to know which of the two brakes is better, in order to take an informed decision.
Disc brakes are way better. They have no self-servo effect like drum brakes. In disc brakes, the braking force is linearly dependent on the brake application force and also on the coefficient of friction.
In contrast, on drum brakes, extremely small variations of coefficient of friction can mean huge differences in braking force. Usually, the drum brakes are not exposed to elements, but if you manage to get some water somehow there, it can be harmful. Furthermore, the braking force is not a linear function of the brake application force.
Choose discs! There is no reason to choose drum brakes unless the self-servo effect is mandatory for a reasonable braking force, like it is on big rigs. Especially on a two-wheeler, I would prefer disc brakes, because the ability to control braking force on two-wheelers is more important than on cars.
The primary faults of drum brakes are they need to be adjusted on a regular interval and brake fade. As the brake shoe material wears the distance between the drum braking surface and the brake shoe increases. Hence the need for mechanical readjustment. Disc brakes are self adjusting. The fade issue appears as the drum gets hot from the friction of the brake shoe. As the drum get hot it expands. This increases the distance between the shoe and the drums' braking surface. This means additional pedal/lever pressure to achieve the same braking force.