Yes, the "Look" is critical -- for this particular character. Hate to pass on such a awesome actor though.
Way too vague for any of us to answer your question.
Let's say that Tatum Channing is the perfect look for the role, but Paul Walker handles the material better. I'd go with Walker, but I wouldn't fault you for choosing Channing.
But what if you're faced with a situation in which Tatum Channing has the perfect look, but Morgan Freeman has auditioned for the role. Time for a major rewrite.