Ok, first of all I want to say that I was seriously stunned when I saw the art in the preview, and even more surprised when I saw the larger version. I even needed a couple of efforts to start the right way. And yet I wonder why. I guess you just got one of the few poses which suit this hedgehog perfectly.
The little image and the larger version as background is common used, but it works pretty well here. Especially your new coloring style is well seen in this one.
You put the shadows great as well. The round curves around the mouth, the hand and the single fingers, I guess you just have an unerring eye for this.
But I got the feeling you didn´t choose a clear direction from which the light comes from. At least the light on Shadow´s chest fur doesn´t appears realistic. On the one hand I can understand that a complete shadowed chestfur would look boring in some way and the light just brings some life into it, but I think it´s better to have a clear point for the light and you should stick to that.
Further I want to say that the colours appear pretty soft to me. I particularly love the eyes in some way. It´s simple made, though it looks great, specific in the little image.
There´s just one thing which bothers me a bit.
Shadow´s left hand appears quite good to me. It´s a little bigger ´cause he points at us and I like how it turned out, but the right hand is just too big, really.
Mayhaps it´s just your style, but compared to his head it´s really to big. I noticed that you prone to make them a bit more little than usual. I saw your last Silver artwork as well and I thought the fingers of the right hand turned out a bit big, too.
But neverminding the size they look pretty well. You really know how to draw hands and fingers in a good position !
And well, my last points just apply some little details and some tips.
First I´d have made the little light effect on Shadow´s nose to the top. No matter which image I´ve seen, the light is always on the top Oo
Of course, the light is good as it is, but I just noticed it. Weird thing though...
The next thing is his forehead right above the nose (below the red stripe on his head). You made a stright line there but one of it hangs always over. I guess it´s one more thing which goes for the style of each artist, but like the nose it looks unusual.
(Ha, dare to be diffrent )
So if it´s your style... nevermind that ;D

And one more serious tip. The little Shadow version looks almost flawless. The colours are darker than in the background, the white frame is seen very well etc. but the lineart around the large version appears boring. It´s great as it is, no doubt on that! But I think a lineart style like ~MintoTheFox has would give the image more detail and expression. As I said it´s just a tip and I think it´d look great, I don´t force you to use it (;D)
Maybe a diffrent background colour than white, ´cause it doesn´t really emphasizes the hedgehog around the eyes, but I already shut up... just think it'd support the vision a bit....

Hope I didn´t disgruntle you, had bad experiences the last days whith critique... though I love the piece, like so many other deviants
I guess the next wallpaper will be Sonic? Can´t wait! Keep going such great works!

S.A-