Zillow was best known for its Zestimate which as we agents know, can not be relied upon as gospel, it does give a consumer a range to consider.
Realtor.com, I understand, pulls its listings direct from participating MLS so the information should be more accurate.
What Realtor.com does well, is provide up to date information that we can count on to be accurate. It also has a great feature for automated emails and an app that is very user-friendly. I feel like Zillow has a lot of information available, but I am hesitant to direct clients to the site because oftentimes it is very inaccurate.