For 50" the KU7500 is better because the 49" X800D uses an IPS panel with poor contrast and black uniformity. The X800D is only better if you want to use black frame insertion in game mode; this flickers the backlight at 100 Hz which reduces motion blur but the flickering may be bothersome.
For 43" the KU7500 is slightly better than the X800D because of its 10 ms lower input lag, unless again you want to use black frame insertion in game mode.
For 43" the KU7500 is slightly better than the X800D because of its 10 ms lower input lag, unless again you want to use black frame insertion in game mode.