A 5:7 AP ratio between a horizontal/vertical of one square and a diagonal movement of one square is pretty close to perfect. The actual number is 5:7.07168, but who's counting. If a unit is charged 5 APs for a horizontal or vertical move into a given terrain, and 7 APs for a diagonal move into the same type of terrain, the different movement costs are fairly accurately reflected. And you get all the advantages of the isometric view over the hexes.