You should use Zillow, especially if you don't mind being wrong.
On the Zillow website, there's a link at the bottom where you can click to find out how accurate they think they are.
According to Zillow themselves, their median error in San Diego is 9.1%. This means that on a home that would actually sell for $500,000, half of their Zestimates would be between $454,500 and $545,500. And half would be outside the range.
This, to my mind, isn't particularly helpful information. Flip a coin - your $500,000 house is either worth between $45,000 more or less, or it isn't.
You know the real answer for determining how much to ask for.
All the best,