Scar was very devious, but lacked the physical strength to take on his brother directly. That’s why he schemed and plotted to murder him rather than face him directly. Because of this, I’d bet on Shere Khan any day in almost any scenario other than poker.
Scar strikes me as the more intelligent and cunning of the two. But yeah, I agree with Austin. In a direct battle, Shere Khan would probably come out on top.