Why does Roark pick Mallory for the sculpture in the Stoddard Temple? What is Mallory's concept for the statue? What is the final result?

Roark wanted the best man available for the job. Thus, his first choice was Mallory. 

"And so I didn't come here to do you a favor or because I felt sorry for you or because you need a job pretty badly. I came for a simple, selfish reason-the same reason that makes a man choose the cleanest food he can find. It's a law of survival, isn't it?-to seek the best. I didn't come for your sake. I came for mine."

Mallory uses Dominique as his model, in an attempt to capture the human spirit...... a sense of exuberance. 

In the end, Toohey gives Dominique the statue.