Decisive means "conclusive," "final," "having the power of putting an end to something": "This was the decisive battle of the entire war." "The lawyer's final argument to the jury was decisive in arriving at a verdict." Incisive means "sharp." "keen," "penetrating": "The last speaker was blunt and incisive in his remarks. In fact, his appeal was so incisive that it turned out to be a decisive factor in the campaign."
Dictionary of problem words and expressions. Harry Shaw. 1975.