Zillow is buying Trulia, its chief rival, for $3.5 Billion in stock. The two companies – both nine years old — have a lot of overlap currently. But after the deal closes in 2015, they will seek to develop two differentiated marketplaces for real estate-related information, which includes house sales, rentals, mortgage and related national and local advertising.

As the acquiring company, Zillow would focus on “top of funnel” awareness advertising. Trulia, meanwhile, would focus more on specific agent-related, final purchase (or rental)- related advertising. According to ComScore, Zillow attracted 83 million unique visitors in June, while Trulia attracted 53 million. Roughly half of Trulia’s visitors do not visit Zillow.

The proposed purchase price, roughly $70.53 a share, represents a 25 percent premium over Trulia’s current stock price. Combined revenues from both companies could produce $721 Million in 2015 under present conditions, according to estimates by Benchmark Research. Separately, the companies estimate $100 million a year in cost savings by eliminating redundancy. Under terms of the agreement, Trulia CEO Pete Flint will report to Zillow CEO Spencer Rascoff.

In our view, the primary goal of the acquisition isn’t to build the one-two punch of differentiated real estate sites, or even to maximize cost savings from eliminating overlap. Mostly, it takes Trulia out as a rival company, and per GeekWire, it also ends apparent merger talks between Trulia and Move.com, the #3 Real Estate site that controls the NAR’s Realtor.com site. (It also isn’t the first time Trulia has considered selling itself. Google apparently was interested in buying the site in 2009 when it was pursuing a major listings effort).

Over the next several years,the effort to differentiate the two sites make more sense than to collapse them into one brand. Such a strategy would be reminiscent of what AutoTrader.com has accomplished with KBB.com; The Weather Co. has accomplished with Weather Underground; and what Match.com has accomplished with the purchase of several dating verticals.

Winning national advertising dollars is especially viewed as a key growth area. Zillow has budgeted $45 million in marketing dollars this year to accelerate that effort. Zillow, perhaps best known for its controversial Z-Estimates, sees a unique advertising market among speculative home browsers, targeting everything from landscapers to auto companies. Trulia, meanwhile, has been less controversial than Zillow in the Realtor community and might be a better brand for Realtors to work with.

Will there be anti-trust issues? Both Zillow and Trulia tend to draw from Realtors and brokerages that are digitally minded in their advertising. Zillow head Rascoff, however, suggests that the market is nascent and represents less than 3 percent of the $12 Billion market in real estate advertising.

We don’t know about that. The reality is that the two companies actually tie up a great deal of the linkages between real estate advertising and distributors, such as the search engines, local media companies and others. But ultimately, it probably falls short of real anti- trust concern.