The Speed Force gives Flash abilities beyond pure speed that Sonic can't match — phasing, time travel, and speed-stealing.
Both are 'the fastest' in their respective universes, making this the ultimate speedster matchup. In pure straight-line speed, they're comparable — both approach or exceed lightspeed.
The difference is the Speed Force. Flash doesn't just run fast — he has access to an extradimensional energy source that lets him vibrate through matter, steal speed from other objects, create time remnants, and manipulate time itself. Sonic's speed is incredible but more 'conventional' — it's physical speed, not dimensional hax.
Super Sonic adds invulnerability and amplified speed, but the Speed Force provides capabilities that raw speed can't match. Flash can literally phase through Sonic's attacks, steal his kinetic energy, or trap him in a time loop.
Speed Force provides phasing, speed-stealing, and time manipulation beyond pure velocity. Flash has outrun instantaneous teleportation and the concept of Death itself.
Sonic in Super form matches Flash's speed with added invulnerability. Chaos Control provides time manipulation. Sonic has defeated cosmic-level threats in his franchise.
Flash takes the edge through Speed Force hax. Pure speed is comparable, but Flash's extradimensional abilities provide win conditions Sonic can't counter.
The Flash also fights
Sonic the Hedgehog also fights