Battle of the tech titans: Ellison vs. Benioff

FORTUNE -- Fierce competitors need, well, fierce competitors. The latest smack-down? Oracle (ORCL) billionaire CEO Larry Ellison vs. Salesforce.com (CRM) billionaire CEO Marc Benioff. Benioff once worked for Ellison, who later invested in Salesforce.com, the cloud-computing software company. No matter: Now the two battle for customers -- as well as bragging rights as the world's most visible software mogul. The rivalry is getting testy: Recently Ellison canceled Benioff's keynote at the Oracle Open World conference.