I personally like the second one better. I love mixing styles - 2D animation with 3D animation for example - but I think it's better when each element is consistant with itself. A flat 2D head on a 3D body feels awkward. I heads have more diminsionalty on the second on so it looks more like they belong together with the bodies.

Before they made me they broke the mold!
http://home.roadrunner.com/~kflach/