All I know, is up til now, they both were pretty accurate, as to what has been happening in my neighborhood. Trulia spot on with average neighborhood value, and Zillow with an honest price range for my actual house. But I've noticed in the last month, Trulia went from about 120K average listing, to 220K while Zillow says my house lost about 10K.
I've got the biggest house with the largest lot, in the neighborhood, you can see the damn thing from Space. Yet Zillow doesn't seem to think my house is worth Trulia's median price. Something don't jive.