I agree with both sides of the argument on this one. Obviously a head coach who has been successful is going to have a better resume, be a more attractive candidate and have presumably a better chance for success. However, they are also going to cost more and have more job options. We may not be able to hire the coach of our choice.
Then you start looking at coaches who have been less successful, and eventually you get to the point where you are looking at coaches who have sort of established a ceiling, ie, they have proven just how good/mediocre they are.
At this point, you start looking at assistants because of the potential payoff. Pretty much every successful head coach was an assistant at some point and got his break. If you are the school who gives that next star coach his break, you are going to get a huge payoff, at a much lower cost salary wise.