I agree with comegetme about getting samples together in the actual kitchen space. The reason why I prefer the lighter one is the pattern. Even though I really like the pattern in the darker one as a single tile, I think that the floor will end up with too many fairly clear lines running crisscross, or if they're laid in the same direction, the lines will never match. I think it would drive me bonkers. Whereas the marble effect tile has a softer and less linear pattern. But it might not be a problem for you :-)

My bathroom looks to be about forty years old, judging by the dated tiles. The tiles are awful, but the shower grout looks fine! If you like the small format tiles, I wouldn’t necessarily assume they can’t also go in the shower.

Maybe some kind of tile trim? Most of them fit half under the tiles to secure them. Or plastic strips like window fitters use to tidy up gaps. Something like this but tile shops will have a selection.

Thanks for the advice everyone. The walls are just plastered at the moment, so I'm thinking to paint them a shade of white first to get a better idea of how the space looks, then decide on some tiles. It's difficult to picture with how it is at the moment! I do like the idea of having white zellige tiles placed vertically, but worried it might look a bit busy if it goes from the countertop to the ceiling.