I'm sick of this comparison. Are you seriously arguing that Xerneas can only go left or right, and never forwards nor backwards nor downhill nor uphill, and that Yvertal can only fly in a straight line up and down? Both can move and rotate along X, Y and Z axes, albeit Yvertal is more at liberty in moving along the Y axis.
EDIT: My annoyance caused me to post before I read the rest of the topic. I'm glad I'm not alone in this.