Grey Definitely.

If you want the grout to fade into the background then white - for peace of mind grey. The grey grout kind of frames the tile so it becomes more of a co-start than a supporting character. I would never ever consider white because I would loose my mind with the maintenance issues.

Make this another vote for grey!

ok, so light or medium gray then?

thanks to both of you :)

Whichever looks better with the marble in general, or the veins in the marble.

Very very thin grout lines. White.