Age old question! Never owned an "A" but stock answers are that "C"s have more protection in a head on collision, are easier and may be cheaper to have serviced, and seem to be better for families than couples.Some like the wide expanse of Class A windshields, some like the shade of a Class C overhead.
Beyond that, a 27-foot class A and a 27-foot Class C are similar in many ways. Much longer Class A's seem more suited to stays in luxury RV Parks, and less suited to rustic camp sites. We often take our 27 foot Class C to local parks,and events, etc., where parking a much longer rig would be more difficult. Most Class A owners seem to prefer longer rigs.