Be cautious in relying on the info given on either site. The actual real estate market varies so much across the country and even a great deal within larger city/metropolitan areas.
I've plugged in a few properties that I have first hand appraisal info on and these sites are WAY off.....as much as about 40%!!! None of them had the correct sq footage of the property and even correct number of rooms and baths!
If you're looking to buy....do more homework about the area with local agencies and tax offices. The biggest problem seems to be in older neighborhoods where they are just doing a neighbor "comp". These kind of neighborhoods can have valuations all over the map and remodels galore that change the value. Only a professional appraiser can give the most accurate info for individual properties.
Good luck!