On the 747, moving flap 25 to flap 30, most of the increased deflection is taken up by the aft flap. This puts a lot of stress on it and subsequent increased maintenance. With the reasonably regular reports of flap sections breaking off 747s, it's invariably the aft flap that falls off. Some operators ask their crews to do flap 25 landings for this reason. However, the two airlines I flew the 747 with both had 30 as their SOP. The only exception was with the 747 freighter. At very light weights with a empty freighter, landing with flap 30 led to a very flat attitude with the increased risk of touching the nose-wheel first, so some pilots used flap 25 in these circumstances.