According to this article, neither of them: http://www.beyond3d.com/content/articles/8/ http://www.beyond3d.com/content/articles/8/ That article also links to

According to this article, neither of them: http://www.beyond3d.com/content/articles/8/ http://www.beyond3d.com/content/articles/8/ That article also links to a paper (http://www.lomont.org/Math/Papers/2003/InvSqrt.pdf http://www.lomont.org/Math/Papers/2003/InvSqrt.pdf) analyzing the initial approximation constant.