The proton on the left is more acidic.
Work Step by Step
In both cases, the charge will be spread between two oxygen atoms, so ressonance isn't the difference. The fluorides will attract electrons from the negative charge from the oxygens on the left, so they will be more stable. The chlorides will do the same to the oxygens on the right. But, since fluorine is more electronegative than chlorine, that attraction will be more intense on the left oxygens. Therefore, the proton on the left will be more acidic.