Moon's.
First off would be the background. The sun in Sun's background doesn't have a clear "trapping line," which is fine in theory because it's like a fiery sun object. But the very distinct contrast between the moon's edge and the starry sky background I feel helps Moon's by giving it a very obvious background. I can barely see that there's a sun in Sun's background, it just looks like a gradient. That combined with the colors of the background makes it unclear if it's supposed to be a fiery sky background, or if Solgaleo is supposed to be "in the sun." Lunala's background is very clearly the night sky with the moon.
The other positive factor would be Lunala's silhouette. The fact that Lunala actually reaches the edges of the box creates a sort of "trapping" effect; there's nowhere to really escape to when you look at it. Because Solgaleo is, in its entirety, in the center of the box, you can sort of escape to the background all around it, and in a sense "leave the box." And the general silhouette of Lunala also makes it more obvious that Lunala's head isn't in the dead-center of the box, giving it a stronger sense of movement, along with the curves of its wings.
