Redfin is more accurate than zillow, They list new listings much faster and the interface is better. I'm in LA and redfin is superior.
But they only have listings for the regions where they sell homes, so they have nothing for most of the United States.
Edit: most markets by aggregate size. That is, they've signed the largest mls'.
Most of the US is very sparsely populated, so I don't think that's a good measure of coverage. Rural areas and smaller cities deserve service, too, but that'll come with time.
(I'm currently employed by Redfin.)