Both look good. I am usually all about grout blending with the tile color, but for some reason I am leaning toward the darker grout. I think it makes the black squares look blacker/richer, and makes the white look brighter, and whiter. The lighter grout provides a more muted look IMO, which is nice, too. Just a matter of which look you envision for the room.

I agree with cat_mom. I usually prefer grout that blends, but in this case I think I like the dark color on the left better. Both do look good, but the dark really highlights the white and the basket weave layout really well.

Exactly what cat_mom and pricklypearcactus said!

I'm going to be the party pooper and confuse you. I like the lighter grout. The darker is harsh on my eyes and i would not enjoy seeing such definition/harshness all the time. I can certainly see that others would like the darker. Just not my cup of tea.

I'm torn, but I think both look good for the reasons already stated. Gray is always a good option, IMO, but the really dark grout does look nice with the small black center.

I am so glad to see a sample board of tile & grout. It is so helpful. Just seeing the tile board I like the lighter grout. The dark one is bold. I think the other elements in the room will need to be considered. Are you trying to have the dark grout tie in with something else? Are you going for high contrast or medium to light contrast with other features in your room? FWIW, the darker grout looks more masculine to me. But, I would love seeing more of your room to help me with my opinion.

My shower floor is this basketweave and we chose a dark gray grout. It holds up with no problems.

I prefer the lighter grout. Depending on the size of the bathroom and the (natural and other) light in there, the darker grout may make the floor appear too dark even with the white tile in the basket-weave.