I own a Model S and definitely would disagree with you here. One thing that is so ridiculously awesome is when you expand a screen to take up both halves. I only use this for Maps or the browser but especially for Maps with Google real time traffic data. It is so convenient when I need to see more area to just double the size rather than zooming out and you would lose this with two halves. Also a fullscreen maps view is the biggest mouth dropper when others get in the car.
Oh, I'm not saying it isn't awesome to use, just the dash at the sides of the bottom of the screen looks weird to me.
As far as phone size, I bought one of the Spiegen cases when I first saw them on Gizmodo like a month or two ago, so I had a good idea how big the 6 would be. I also printed the PDF from ARSTechnica and cut out cardboard mockups of the 6 and 6+ to hold and play with and that helped immensely.