Bioterrorism includes the release of viruses, toxins, and bacteria which goes on to infect the masses, usually with the hopes that it will kill a large number of people. I would venture to say that bioterrorism and nuclear attacks are approximately at the same level when saying they are morally wrong. They both affect the lives of innocent people who want nothing to do with warfare.
Terrorism by its very name is the use of physical violence against innocent people in order to bring attention to some cause or idea, and it is always morally wrong because it is making victims of the innocent and hurting or killing them. Whether it is nuclear or biological, these acts are equally immoral.
With killing, it ultimately depends on how many people were killed, I guess. If the bioterrorism attacks kill more, they're less moral. If the nuclear attacks kill more, they're less moral. But, in either case, they're obviously very immoral. So this question cannot really be answered properly. It's too complex.
Although bioterrorist attacks are incredibly scary, they pale in comparison to the fall out of a nuclear attack. A nuclear attack has the ability not only to kill far more people than a bioterrorist attack, but it will have effects on the land it is detonated long into the future.