I think Sten would win. I don't know if it was a hint or not, but in conversations between Iron Bull and Varric, the mention how the new leader of the Qunari, The Arishok, had no horns this time around. It's very possible that Sten surviving the Blight may have worked up the ranks to become the leader of the Qunari.
As much as I love Iron Bull, Sten is very rigid in his views he is completely commited to the Qun and has never wavered from that. I feel like he has a lot less to lose that Bull (well depending on what choices you make) and therefore would fight that little bit harder in my opinion. He's also older, so more experience, and there's a theory he could actually be an Arishok now, making him a hell of a lot more powerful I think by now. But on the flip side Iron Bull does have more to fight for, so if you look at it that way perhaps he would win if he was fighting to protect that. He's definitely a more likeable character though, seeing as Sten murdered a whole family of farmers and is generally a bit of an ass. That's my opinion anyways haha..