I actually agree with folks in that the second sketch is done a lot more realistically, at least in terms of features and general physiognomy. But yeah, as you mentioned, it doesn't really resemble Steve too much. The thing you have to keep in mind is that Steve always has to look bigger than the average athlete. He's a superhero - so he has to look larger-than-life. I believe Stan Lee himself made that comment in Buscema's 'How to Draw Comics the Marvel Way'.
Try increasing the mass on his shoulder blades and his neck. In your first sketch he looks muscular and defined, but in the second one he looks like a skinny cosplayer. The other improvement I'd suggest are the eyes. Specifically, the eye-holes of Cap's mask. Try making them more angular, like in an angry expression ala Epting or you can try a more triangular look ala Garney. Remember that when it comes to a mask like Cap's, a lot of his facial expression comes from the brow.
Aside from all that, I do think it was a solid effort, BFJ. Nicely done, and I hope my pointers were constructive :)