Works by younger artists are well represented in the show. - This is talking about the amount of "works" that younger artist has in the show. The sentence therefore means that there are a lot of works in the show that are made by younger artists. Works by younger artists are well presented in the show. - This sentence says that the works made by the younger artists are looking good. This is therefore not talking about the quantity of works, but rather the quality of works. On the question of whether one is better to use, the context, in which the sentence is used, needs to be clear.