White, will make it will pop. Grey will soften the look. If you go white, be prepared for more work to keep it nice. If you want to try it first, get some colored paper to put behind a sheet of the tile as a mock-up.

Both grout being very light will not make much of a difference. I'd go with the grey for easier maintenance.

my vote is for what the predominate color of the tiles is to make them look less busy.

Never white if you can help it it is always hard to keep white.