The natural light and shadows, the subject's detatchment from the scene, the "in action" feeling, the background free of distractions all work to make this an excellent portrait of an athlete. Forgive me for saying so, but the second one is the exact opposite in all these aspects, and feels a lot more like a nice, sharp, colorful snapshot.
Odd thing is, since there's no context, either of these shots could just be some dude in a rugby shirt. :) The first one, though, feels like it's a shot of an athlete.
I like the sepia effect of the first, too, but I don't think it would work as well for the second... and if the sweat in the second one is important to you, consider cropping very tightly to emphasize that. Also, the subject seems a bit wall-eyed... perhaps there's another frame that de-emphasizes that. And maybe there's another frame without quite so much background clutter... in similar situations, you might try your camera's lowest f-number (largest aperture) to throw the background out of focus and concentrate attention on your subject. You might even be able to get away without using flash, because you could use a faster shutter speed. I took the liberty of ROUGHLY simulating how that might look:
It's easy to make suggestions after the fact, not knowing the circumstances where the shot was made, but if you have a little time to maneuver the situation, it's usually worth the effort. For instance, I might have had the second subject sit up on the top bleacher bench and shot parallel to the fence, taking advantage of the natural light and getting some shadow play in his face, and you could switch off the flash. Rather than smiling into the camera, maybe a more casual pose that suggests an athlete tired out from a hard game, even if you shot from the floor up to him to emphasize his power and get a more dramatic angle. The roof of the grandstand could serve as a good backdrop that would almost certainly go out of focus.