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 ofconfirming ...
