How do you pay your contractors?

[Joseph Weisenbloom]

Are you paying your contractors by the hour, or by the project, or by square footage?

If you are paying by the hour how much do you pay per hour? 

If you are paying per project how much do you pay per project?

I've been struggling with finding the best way to pay my contractors. If I pay per hour I usually pay less but the progress is slower. If I pay per project I pay more and the progress is fast. Is it normal to feel buyers remorse if I pay per project and they complete it very fast making their per hour rate really high?