edit poly is better for modelling things because it has more tools, its more powerful
for unreal it doesnt matter anyway, because at the moment you export and import it in ued it will get auto triangulated anyway
thats the only real different with an edit mesh, edit mesh makes every polgone a triangle, with an edit poly you can have any shape, but that difference will disappear on export
mostly i start in edit poly, and by the time im done i convert to mesh so i can check if he triangulated it all correct and to see the true polycount