The preference of the Xbox and PlayStation 3 controllers, generally lies with the type of Video Game you are playing at the time. For those who enjoy playing Fighting games, the overall consensus is in favor of the PlayStation controller- if not an Arcade Stick. Blocking with the beveled D-Pad of an Xbox controller on Street Fighter IV can become problematic. My inputs for Tekken Tag Tournament or Persona 4 is extremely hard since the buttons for the Xbox are so stiff.
Its generally agreed that the Xbox controller is best for FPS games, but I personally like the PlayStation controller, overall. Bigger hands may prefer the Xbox, while smaller hands may prefer the PlayStation. Perhaps there is no better or worse, just a matter of preference. In any case, head over to Etokki dot com, for those who wish to buy converters to use the Xbox 360 controller on the PS3 and vice-versa.