Staying White or Going to the Darkside?

I've been searching for subway tile examples for a client today showing what a difference grout color can make.  When we were shopping for tile last week we looked at a lot of different options but settled on a clean and classic white subway tile to go with the rest of her clean and classic kitchen.  When it came to grout though, she didn't know of any other options other than white.

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Without being able to visualize the difference she didn't know how she felt about a light gray or something even darker.

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So what do you think?  Which grout works best?  The white blends everything together for a more seamless look white the various shades of gray make the tile and pattern pop.  So what do you think?  Which grout works best?