Editing: Charging hourly or per project/milestone?

Hey, so I'm about to start outsourcing my work to editors and have a querie about charging. Should I charge the editor per hour or per project?

I'm worried that if I price an editor for 2 days (14 hours), they could do bugger all editing and when the 2 days is up they still involve me for 14 hours worth of work (though theres still heaps of work to get done).

Or charge them a flat fee (taking into consideration the scale of the project and ofcourse at their rate) to get x amount done and its up to them how quickly they want to finish it (within a rough deadline etc).

Thoughts?