Oh gosh, great concept. I really like your style too, especially the shapes of the wings, hair and clothing. I also like how you only added a hint of flesh tone on the angel/devil, so the viewer can tell they aren't real even though the poor boy can't.
The expressions are great; the boy's face is very troubled, the angel soothing and the devil seems to be feeding him feelings of doubt.
Anatomically, everything is quite good, but the hands could use some work. Aside from the one the devil is holding to her face, they seem rather small and stiff.
I think other than that, the only thing that could add to the composition would be if you went a bit darker with the shading, as there's a lot of white all over. But from a quick look through your gallery, it seems to be your style to have lots of white, and it still looks great this way too <img src="e.deviantart.net/emoticons/s/s…" width="15" height="15" alt="" title=" (Smile)"/>
Great job on this! I hope you are going to be continuing this series, I love the concept. <3