The two authors, Demetrios of Skepsis and Polemon of Ilion, are native to the Troad and write books about this landscape (Demetrios’ Τρωικός διάκοσμος and Polemon’s Περιήγησις Ἰλίου). There is however an important difference between the two scholars in their approaches.
A first hint may be given by the fact that Polemon is considered as an important source for Pausanias and his Periegesis, whereas Demetrios is mainly quoted in Strabo. W. Hutton gives another interesting attempt to explain the different scopes of the two authors. He emphasises that Polemon, among other authors, focus on the landmarks created by humans (monuments, shrines, inscriptions…) and the stories linked to them. Strabo, and maybe Demetrios, is more interested in the topographical form of the landscape, wherein monuments could be built.
See W. Hutton, Describing Greece, Cambridge 2005, 241-272