While I am a 100% fan of the PS3 and it is my console of choice I also own an XBox so I can play with friends and family. Due to the stupid positioning of the Analog stick on a PS3 controller (in my opinion, the only failing of the PS3) I always have problems switching between the two consoles. The PowerA controller is the perfect solution as the Analog stick is in the correct place. Being slightly larger than the PS3 controller I also find this a better fit to my hands. The PowerA controller is also covered in a very nice tactile covering which also makes it feel good in the hand. It also has back lighting so you do not need the light on when you get confused over which buttons to press. Not to mention full 6-axis control is supported and force feedback.
The only drawback to the PowerA controller is that it uses its own WiFi dongle rather than the proper built in WiFi supported by the PS3.
Overall, the best controller I have ever used.