First, the A. clyton is a male, and the A. celtis is a female, so that is why the over-all shape of the wings is different (and it's a good male - female comparison, even if different species).

Also, I wanted to comment that it is important not to focus on the color hue, over-all darkness, or "strength" of the dark markings. Look instead at the eye spots on the front wing (or lack of them), and look at the placment and shape of the dark markings (especially toward the base of the wings. A. clyton is probably more varied in coloring, and is more likely to have very dark wings in some individuals, but not in all.

They are very similar species, but not difficult to tell apart once you get the hang of it.