Both Zillow and Realtor.com get the bulk of revenue from real-estate agents, who use the websites for marketing, customer management and lead generation. Leslie Ebersole, a real-estate agent in St. Charles, Ill., moved all of her online spending to Zillow in 2013 because it offered better support than either Trulia or Realtor.com.

“I’m a card-carrying member of the NAR,” said Ebersole, a Realtor since 2005 who leads a team of six agents that sold $30 million of homes last year. “I’m fine with NAR promoting the Realtor brand with Realtor.com. But as an agent growing my business, that doesn’t help me against other agents.”

Swanepoel, the CEO of Swanepoel T3 Group in San Juan Capistrano, Calif., said the Zillow-Realtor.com competition will help buyers and sellers of homes by spurring innovation and improving the quality of market information.

“The one company is perceived as younger and dynamic, the other as established and conventional,” he said. “The beneficiaries will be the homebuying and selling public that will have robust apps and tools and an ever-increasing database of property information.”