Round just doesnt make any sense. We need to look at this in comparison to everything else, put it in perspective.

What shape is Overworld? There is not actual curves to the shape of the world. Following this pattern, it would only make sense that Overworld is in fact a cube. (If it isnt completely flat)

Making the sun and the moon round is completely nonsensical. It doesnt fit with the theme or style of minecraft whatsoever, which is exactly why it should be square, if not cubed.