Why is benzoic acid a stronger acid than acetic acid?
It is all about the electron withdrawing/ donating nature of the acid's functional groups.  The more electron withdrawing the group, the stronger the acid will be.  The more electron donating the weaker the acid will be.  A phenyl ring is electron withdrawing, while a methyl group is electron donating.  I don't know why resonance would destabilize an organic molecule or ion.  if you are referring to the carboxylate donating electrons to the phenyl ring, I don't see how this could be possible and not violate the octet rule.