Personally, I think that the colored version is beautiful, but probably doesn't look so well as an card artwork piece - I think it's the positioning of the sun, moon, and 'dual'-winged object that makes it seem more of a poster rather than a visual for a spell or creature (the final product makes me sad, because those words ruin the art completely
). If the sun/moon was closer to the Yin/Yang symbol, I think it'd work.
The black and white version is simple and clean, but it would be too dull for card art even with the added moon, sun, and sky effects. If anything, your 'Duality' art is super cool - I would LOVE to see this done in colors of red/blue, or
&
if you know what I mean.