White stands out more. I think there are better colors to choose than white. Go to any good tile store and they often have sample sticks which you can freely take to lay along your tile and see which you prefer.
I believe in being practical--a belief that drives my wife crazy. White grout in almost any application looks beautiful immediately after done and for another 15 minutes. I go for the color the white will look when it's dingy: gray. Gray is even better than black. I had black grout in a shower once, and even it eventually faded to gray. So white or black, you ultimately get gray anyway, so why not start with it.