One of the problems of policing is that you can't be everywhere. You have to make some decisions about priority. You can police the suburbs, but if all the crime is downtown that may not be the best use of your time. On the other hand, if you spend all your money downtown, you may find the criminals moving their operations to the suburbs.
The same is true when trying to figure out how to set up a national defense against chemical and biological terror. It may be--in fact, it is--the case that some of these chemical and biological weapons present a greater danger than others. Rather than building up your defenses against all known agents, it's a better idea to determine which ones are the largest threat, and optimize defenses against those before you worry about the smaller-scale threats.
How do you judge the relative danger? There are a few useful questions. Just how deadly are chemical and biological weapons? How hard are they to make? If a terrorist wanted to get his hands on some, would he need specialized tools, or are there "dual use" technologies that could do the job?
The Congressional Research Service (CRS) has written a report on chemical and biological terrorist threats that seeks to answer exactly those questions. It's apt to be driving the debates in Congress for quite a while. Follow the link and read up--you'll be better informed, and better able to keep your politicians from playing games in favor of doing what really needs to be done.