The Flash is faster, just by a hair.
The Superman-Flash Races
The article covers the 6 races (that were finished to some degree) between the Flash and Superman that have taken place in DC Comics. Of those six, two finished in ties and four finished with the Flash winning (Wally West took two, Barry Allen and Jay Garrick each took one).
An interesting comment in the article:
Admittedly, a Flash fan-site is a biased source of information, but it's been said by multiple DC writers that the Flash has, potentially, the most power of any DC hero. If Superman were every bit as fast as the Flash, then the speed-force wouldn't have any edge over Kryptonian physiology.