Personal preference - they both look great! But the black grout is more of a statement (which i love!)

Thanks everyone for their comments! Update: husband has 'over-ruled' on this one. Forgot to mention that the vanity top will be Night Sky Ceaserstone (ie black). So he wants the dark grout. We will definitely be using mould inhibitor and ensuring everything is well sealed. It's all his fault if it looks horrible ;) Will post photos once fully complete. (I am now a little scared...)

I have just laid grey floor tiles and white ( with a very subtle pattern) and I have used the same light grey grout for both. White grout is a nightmare to keep clean.

Sounds like a major hassle to me! White grout paint is usually for cleaning up grout stained by mildew, etc, not to go over a completely different colour. I am really surprised that your tilers did not consult with you about the grout colour before they went ahead. If you are really unhappy, and they acted without permission, you might be able to get them to redo. Otherwise, you may have to live with it and eventually wont notice it once the space is in regular use.