Neighbors house just went on the market last week and is under contract for $440,500. Neither site seems to acknowledge that it was listed for sale, yet.
Trulia estimate - $404,000
Zillow estimate - $441,599
Just one example, but leads me to believe Zillow has better equations at play. At least for our area.
The thing both of these sites really miss on is upgrades and features. Two identical houses side by side could have same sq footage and proximity to 'stuff'... but if one has a deck, patio, gourmet kitchen and finished basement, it could easily sell for $100K more. However, on zillow/trulia they would be listed as identical prices.