Make sure you want black on a shower floor - it will show every drop of dried water...

Black grout for the floor tile for a seamless look.

I wouldn't do either - the floor will just look like solid black. What's the point of getting a mosaic herringbone if you aren't going to show off the pattern slightly? Go a little lighter and get a grout that doesn't require sealing - they are usually stain resistant.

If those are the only 2 options, I would go with the lighter one.

I assume you won't be installing the herringbone tile on the Ditra because it has a 2"x2" minimum tile size spec.

Jeff-
No. Large format tile will go on the floor.

@msto1129 Good. As far as grout is concerned I don't like there to be too much contrast between the tile and the grout. It exaggerates any irregularities and I think can look a little "cheesy". You've made two good choices and I'd go with the dark grey. But before you decide just lay down some tile and grout it and see which you like best.