Of the three, I'd take the Glock 21 over the other two. It has a track record that the other two just can't touch. Of course, they haven't been around as long. I don't think any gun has been tested as much as the Glock, and it always comes out on top. Accurate, and easy to shoot, it's even my wife's favorite gun. I've heard the new ones even have a slimmer grip, too.
I'm sure the Springfield and S&W will probably turn out to be fine guns, but the next ten years or so will tell the story.