从意思上说,这两个词组没什么区别,都是“……的证人”的意思
用法也基本相同.唯一的区别是 of后面必须跟名词形式 而 to后面除了名词形式,还可以跟动词原形
Sheikh Amer was the only person who came forward as a witness to confirm the government account of the bombing.(来源：WSJ:Nine Libyans Killed in NATO Attack Are Buried)
= Sheikh Amer was the only person who came forward as a witness of confirming the government account of the bombing.
个人认识,仅供参考.