Stock plugs

The manual offers two different size plugs for use, what is the difference and which one is better?

[Moto92]

Stock Honda plugs are NGK. DPR8Z is the standard plug, while DPR9Z is for extended high speed riding. More than likely it will be best to stay with the standard plug. From what I was told, the extended speed plug is a colder plug. It would be used in desert baja races for example, where the engine is constantly ran wide open.