It appears to boil down to these takeaways:
- Zillow and Trulia have incomplete and outdated Multiple Listing Service (MLS) data on their sites at an approximate 15% error rate for some/most of that data (Trulia wants more specifics)
- Trulia and Zillow have more For Sale By Owner (FSBO) and other "listings" not listed via the MLS.
Therefore, while both Zillow and Trulia are naturally trying to improve their MLS data showing on their respective sites at any given time, it appears that when searching for MLS-only properties for sale, a site such as ZipRealty would be the more logical choice. The converse is also apparently the logical choice. When searching for potential deals that do not show up in MLS current data, use Zillow and/or Trulia.
Here's the full article: Accuracy of Zillow, Trulia listing data under fire again | Inman News.