I like the light - it unifies it more with the larger tile.

Depends on if you want to frame each one of the larger tiles or make them look more like one large surface. Since you have the mosaic tiles that seems to be what you would want to feature so I would go with the lighter grout.

I love the mosaic, and want it to be the focus. Great frecommendation to keep the light grout for a more unified background. Thanks for the input.