Since Janelle Monáe burst into pop consciousness a few years ago with her alter-ego Cindy Mayweather, robo-chic became officially a fad. Dainty Terminator claws were cropping up on various performers. And in the comfortable silence of someplace familiar, someplace still, the members of LaBelle wept. Now Christina Aguilera has jumped on the spaceship assembly-line, right down to the sleek, tuxedo-y touches that Monáe uses. So the question must be asked: Who is a better robot, Christina or Janelle?