an ancistrus is a type of plec! Ancistrus is a bristle nose plec, meaning it has bristles on its chin, and around the mouth side of mouth. A plec is an armoured member of the catfish family, belonging in the family Loricarridae. A syno is a catfish form the family MOCHOCIDAE.
Geographic range, anatomical structure, dietry needs, dental structure. You name it! The Bristlenose and Pleco are Loricariids and preside in the Americas, and the Synodontis belong to the Mochokidae and preside in the African continent. Some of the Mochokidae can cling to rocks with their lips not unlike a Loricariid (Syno. Brichardi) because they too have adapted to fast flowing water. While some Synos (Polli, etc) are lake fish so root out crustaceans and other small animals in typical manner. Your question really is like asking whats the difference between, say, a Shark and a Ray? They have simply evolved differently according to habitat, diet and anatomical needs.