You have normal human eyes, if you ask me. You are noticing on the 16:9 that there is more vertical image than your eyes are used to "processing". Human eyes tend to focus on a single vertical point, but are used to having left and right peripheral vision. That is why a wider image seems less straining than a taller image.
I have a 104" 16:9 screen that I was watching from 12 feet, and I was OK with it, but mostly because my movies are in a wider format than that. Now that I am in my new room with about a 14' viewing distance, 16:9 material seems better on the eyes.
If I was doing it all over again, I would go with a wider format screen and lose a little 16:9 image size.
