Not a pro, but for a modern design aesthetic, I think stacked tile would make sense. We did that in our kitchen with aqua glass tile and have been very pleased with the results. I think your kitchen will be absolutely stunning.

I also vote for stacked however have to ask how you are resolving the overall height and transitions around the windows? That coordination or lack thereof can make or break your goals.

Also not a pro, but stacked seems right for the entire backsplash. Let’s hope some experts come here to help you navigate the installation on that wall.

I like stacked but really dislike those shelves on the window wall

personally i have done both and think that you can go with either. a big piece of stacked is making sure that your grout lines work with the layout and tile that you have. i like a subtle grout line and color so that the eye is on your tile and not your grout lines, especially since you already have a lot of contrast going on.