A closer comparison would be gorditas vs arepas, which are very similar in construction and use. The filling for a pupusa is enclosed before cooking whereas with arepas and gorditas, the patty is usually cooked first, cut open, then filled.
However, pupusas and Mexican antojitos primarily use nixtamalized corn (ie, Mexican-style hominy). Arepas, though, are made with a pre-cooked and re-dried cornmeal, sort of like finely texture instant polenta. The flavor, texture, and nutrition are all different because of this. Masa harina (eg, Maseca) would be similar to masa arepa in that it's cooked, then ground, then dried, except it's made from nixtamalized corn. As opposed to arepas, there are also cachapas, which are made using fresh corn. Arepas aren't entirely unlike a johnny cake or corn pone.