I do like the picture below of the honed marble square tiles, they are chunky enough they don't read as subway. The material has an expensive and classic look, just as your counters do.

What areas do you want to tile? Having lived with no splash all summer, do you find the walls got dirty or hard to clean? I really like your current aesthetic, it is simple, clean, and interesting line between modern and traditional, it feel sophisticated but not stuff. It would be easy to tip the whole look of the kitchen with backsplash. I would paint the area under you cabinets the same color as your walls and use a splash only behind the stove.