Retained AOR or project fees? Which one is best?

retainers vs project fees

While I have previously shared thoughts on comparing rate cards with retainers in 2008, ways to improve retainers in 2009 and most recently the considerations before moving from retainers to project fees in 2016 , I have not actually provided a comparison of pros and cons for advertisers and agencies.
Yet in the past five years we have had an avalanche of advertisers asking us for help moving their agency remuneration from retainers to project fees.