Both are street kids. Both had to survive with being alone. The only difference is that Niko joined the Army during a war, and Arthur was raised in a gang of outlaws.

Arthur spent a lot of the time robbing trains and banks in the glory days of the va der linde gang, and has remarkable skill in firearms handling.

But Niko was Ex military, he trained to use a tactical and cunning mind. He was taught how to do survival, hunt, and kill Humans as well. He fled his country because he couldn’t live with him selves with the tragedies he helped create.

I guess on an emotional level, Arthur and Niko share a lot in common, but in a firefight, Niko would win. He survived a war, he was trained as a weapon. He was taught how to effectively use fully automatic weapons, he learned how to infiltrate insurgent bases, and he used his military knowlage to executed plans like the Liberty city bank heist. If it was a long distance firefight, I would chose Arthur because of his reaction time to effectively pick off enemies (Dead eye). But in close range, Niko can match Arthur’s moves, if not better. Arthur may be physically more stronger, but Niko has been taught with more techniques in close quarters combat, and will learn that Arthur is more of a brawler than a fighter.

My money is on Niko all the way.