They're for different tasks. The Model 24 is an offensive, or concussion grenade.As the name suggests, concussion grenades use the overpressure from the explosion to kill rather than fragmentation. As a result, they have a smaller area of effect. These are useful when advancing, as troops will be less able to take cover from fragments. The stick of the grenade allows it to be thrown further, helping with this. The Mk 2 is a defensive grenade, relying on fragmentation. As it throws small metal fragments over a wide area, it can't really be used except by those in cover. The Allied equivalent to the Model 24 would be the Mk 3, while the Germans produced a fragmentation sleeve (the Splitterring) to allow the Model 24 to be used in a defensive context.