I'd go all-in for Cowher. The reason for me is simple. In my opinion, the Chiefs had their best defenses in the early 90s with him as our DC, including getting the best out of Derrick Thomas. Ever wonder what he could do with Hali and Houston? That entices me greatly.
One thing that would be a guarantee with Cowher is that the team would NEVER be soft, which is almost a guarantee with Crennel.