Many factors contributed to the choice of EV3 rather than NXT.
Perhaps the most obvious is that I wanted the robot to be able to solve the cube reasonably quickly. MultiCuber 999 can solve the 9x9x9 in about 33 minutes which is faster than its predecessor MultiCuber 777 could solve the V-CUBE 7. This required more motors to perform more turns with the heavier, slightly stiffer cube significantly faster than before. Since the EV3 has four motor ports compare with three on the NXT, two bricks support the eight motors that are used to achieve this speed.
Another is that I used this robot to prototype and debug some of the communication and motor control code for use in CubeStormer 3 (that can be seen in the background of the video).
(Maybe I should post a link to the CubeStormer 3 "trailer" video too?!? )