It will vary from state to state.
Each site has a certain % that is automatically picked up from the local MLS. So, some sites like Zillow will be good in some parts of the country while Realor.com might be better than other parts of the country.
Zillow is also good at aggregating FSBO and other private sale data. Realtor.com doesn't show FSBOs.
Zillow will mark homes as preforeclosures if in default. Realtor.com does'nt.