Glass is more practical for clean-up, especially around the cooking surface where sauces or oils could penetrate marble.

Marble would be a gorgeous option. But there are a few maintenance requirements when using a marble. It is a soft stone and susceptible to etching. I would recommend a honed marble over a polished, you won't see any etching in it. You also have to seal marble and reseal it over the years, but since you have granite counters you'll be sealing your counter top as well. Glass is a much simpler maintenance, just seal your grout. So it's really up to you and if you're ok with the up keep of marble or if glass would better suit your habits.

personally i would choose some solid piece. i want a smooth surface...no indentations. the less grout to clean the better!

I agree with Johanne. I think the polished carrera in herringbone would look really great in this room.

A 3x12 or 3x6 Subway tile would be a nice fit

I would choose the marble. You are lucky you have such a large kitchen, but because of that there is already a lot of white. I wouldn't want to see a white backsplash. (or if you did do white I would do something with texture like a beveled or other 3d tile)