Both can go faster than light and “therefore” break through dimensional thresholds or go backwards in time; at that point actual measurement is kinda pointless. However, Flash is bound to the Speedforce (of whatever dimension/timeline/etc he’s in) and when that’s shared with any other speedsters, he becomes less capable. Sonic does not seem to share this “weakness” therefore would be considered faster.

This is the same argument for why Superman is actually faster than Flash, too. And ignores the use of chaos emeralds.