Redfin is pretty different from Zillow, since they directly sell homes. So they will have better/faster listings directly from the MLS.

But they only have listings for the regions where they sell homes, so they have nothing for most of the United States.

Most Americans currently live in areas served by Redfin.

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.)

Zillow has had direct mls feeds in most markets since the spring.

Edit: most markets by aggregate size. That is, they've signed the largest mls'.