Is there a true difference between those two models though? In the end, the client has to foot the bill, no matter how the money is allocated between commission and candidate.

As a recruiter, you could have a similar model for contractors and promise a fixed % of the daily rate. That way, the higher the rate of the contractor, the bigger the commission for the recruiter.

However, I do see your point and I notice most recruiters not being very transparant about their commission, nor do they want to give it up & work with a fixed fee instead. I know one small agency who's very open about their commission and promise to only take a cut for the first year. I hope eventually other freelancers will wise up and choose those types of recruiters over the more shady businesses.