just wanted to make this for the sake of showing differences. Atra is a clone of Shadow, but further "grew" on his own physically with the help of Dr Eggman, so he developed into having slight physical differences.
Again, he is more physically strong than Shadow, but is not as fast and cannot use a chaos control like Shadow (he can create short destructive bursts using chaos energy though)

Atra is supposed to be more physically intimidating than Shadow because he is also a big opposite of Shadow, in that he is more loud about anger and hate. In short, he is what Shadow would've been is Shadow had continued to stay under Eggman and was manipulated into evil.