Pietersen has 22 Test match hundreds and 28 fifties at a strike rate of 63. He has an average of 49.56. His average against Australia is 49 also.
Clarke has 21 Test match hundreds and 22 fifties at a strike rate of 55. He has an average of 51.72. His average against England is 37.
So, all in all, both top drawer players. Lets not be needlessly stupid and start dismissing the talents of either simply because they play for the opposition.