It looks like you already have a short upstand to the underside of your window so splashback would look great to the underside of your wall units. I think a bright reflective toughened glass would look perfect and nice and easy to clean. Try and choose a bright or light colour to lift the space and the reflective finish will help make the room feel bigger and is easy to clean too. Only pick a colour you have always liked and a colour you often wear and you wont get bored of it.