Having done both, I can say that I prefer the per-project method for the reasons above, but the problem there is when workload exceeds cost and time estimates,

Having done both, I can say that I prefer the per-project method for the reasons above, but the problem there is when workload exceeds cost and time estimates, and it's either ask the client for more or grind through a project you feel is undervaluing you. Not a solution, just two cents.