A dirty bomb would have a pretty limited area in it's effects, but Ebola could spread very,very,quickly.
This is totally true. The problem with Ebola is that it lacks your "money shot".
A dirty bomb would kill, at best, 50,000 people. However, it would kill them all within a very short time frame. It would be shocking, and cinematic.
A viral outbreak would take far longer. The curve would be exponential, eventually wreaking an incredible havoc, but in the early years the disease would remain external; something that happens to "other people". See also: AIDS in the early 80's. By the time the disease went mainstream, there wouldn't be much the government could do anyway (assuming we're dealing with some incurable super-virus out of a scifi scenario; assuming another AIDS-like epidemic, I imagine the methods of prevention would be discovered quickly, and spread by Federal, State and local governments in tandem).
For jack-boot, old-school, black stormtrooper martial law to exist, the mass majority of people would need to believe that the police-state was able to protect them. That makes sense when bombs are going off, and the bad guys are out on the streets somewhere. But when dealing with a disease, vaccinations, education and healthcare can be instituted far more easily, and much more cheaply, than relying on martial law.
Besides, what good is stormtrooper armor and 20' tasers if an air-borne virus is on the loose anyway? Would the agents of tyranny be just as susceptible, or are we assuming "they" have the secret cure, and are inflicting this on humanity just for kicks?