Fantastic stuff. I can't comment on the accuracy of your sentinel shading, all I know is that it looks very, very pretty.
In regards to the stance, however, I have a few thoughts:
- The lower stance looks very unnatural, because he has a very wide base, yet his legs are relatively straight, so while at a glance he looks ready to pounce, it actually just looks very awkward at closer inspection. Even with superhero powers, I'd expect him to bend his knees slightly before lunging forwards.
- The upper stance looks more realistic, because it looks like a normal wrestling stance (google it and you'll see what I mean). Maybe that was already your inspiration. It makes sense, because a lot of wrestling styles feature explosive leaps forward, so they're stood very low, almost like a sprinter.
- While the lower stance is less realistic, it's ten times cooler than the more realistic wrestling stance. It looks more dynamic, more badass. Especially because of the arms. Maybe you could do something interesting with the arms, while still having his rear foot ready to push him forward, in a realistic fashion.
I'm probably going to totally eradicate any sort of credibility in this post by doing these crappy edits, but here's... some crappy edits. But even disregarding any of my ideas, I still think the knee of his rear leg needs to be pointing forward to make any kind of explosive forward movement more realistic.