I've also found the clients who insist on fixed-fee contracts to be very demanding and petty. They tended to interpret the specs liberally. It's not worth the hassle. Trust me.
When I'm the client, I feel fixed-fee gives the contractor an easy backstop/fallback position - they figure they can half-ass the job and use the additional time to scout for more work.
When I'm the contractor, it's the same, but in reverse - fixed-fee makes clients feel like they can demand the moon beyond spec at no additional cost.
Hourly/daily rates make sure that BOTH parties have something at risk, which tends to be keep everybody in line. The contractor's at risk of the client firing them. The client's money is at risk if they keep demanding endless revisions.
Actually a good lesson for business in general.
I really, really don't like working hourly - I'm not selling my time after all, but my services.
the benefit is that my coder who wrote the tech spec should be very comfortable with what is needed and i get to sell a lower risk $$ to my managers.