Who do you think will win: Zillow, Trulia, or ????

It is my belief that in the next few years a nationwide multipurpose real estate website will evolve to become a major player in real estate sales. At first augmenting, but eventually displacing local Multiple Listing Service websites. I see this service site enjoying kind of a natural monopoly similar to Facebook, Google, Amazon or Linkedin. Currently I know of two such candidates that exist for this role, Zillow and Trulia, both are publicly traded companies. There are probably other players in this arena I don't know about. I expect one will become the Facebook of Real Estate, and that the others will become the myspaces and etoys of Real Estate. I do not use either site much myself since I use a local Denver Realtor site.

My question is for those that have used Zillow and Trulia a good deal, (or brand x I don't know about). Is one site stronger, better, more useful, more reliable, etc.. than the other? My question relates to my desire to make a stock purchase of one of these companies (yes, I know that I am allowed to purchase more than one).