Okay, which head placement is better?
Left, looks more natural to me.
Definitely the left one. The right one has its head on his left shoulder rather than on the neck.
The nose looks a bit too flat at the front. Also, something looks off about his feet, they seem to face slightly too far to the left.
Left one.
Outside of that issue, move the top of the nose down a pixel. Second, things might look better placed if you shifted everything above the feet up and to the right a bit.
Oh, I wasn't planning on continuing this for a while, but I'll keep that in mind if I do.
The shoes need to be a little smoother.
those cheeks have some extreme depth to them, almost too much. I know Koopa's snout dips in like that, but the way you made it, it appears almost like there's a crater there