The main question is always who is the star and who are the backup singers. If your counter is the star, then you don't also want a "feature wall" of mosaic competing for the spotlight. If you don't care that much about the vanity and top, then maybe a flashy tile would be great. Otherwise, either use a low backsplash of the counter material or a medium backsplash (say, up to mirror(s)) of a plain backsplash tile.

Thank you for that advice! If I go with the Statuario Nuvo, it will definitely be the star. The Bianco Drift is more subtle, but I still think I like simple look of having the same material for counter top & backsplash because the vanity itself will be in walnut with some interest.

How high will the backsplash be? Do you know the mirror you will be using? Without knowing anything else I vote for the quartz backsplash.

Since you mention being out of style, I question the desire for a mosaic "feature wall" do you have a sketch of what you have in mind?