We have found that project billing is much better able to meet the needs of agile project development, both from our, and our client point of view. 

We do offer much better hourly rates for our clients that have us on a contract managed services account. Not only do those clients receive priority service, they get an hourly rate that is up to 50% less than non managed services clients. Non managed services clients also must wait in the service stack for the next available time slot to be serviced.