Hello, and thank you for the question! To be honest, the grout color really depends on the tile you're using and what look you're going for. If you want the grout to stand out, go for the white. Warning: stark white tile takes a bit more upkeep due to water/soap stains, etc. If you want the grout to not stand out as much against the tile, go for the gray, which would be slightly less maintenance to keep bright!

Hope this helps, and best of luck!