Hi Murrayjames, from what I understand, this is the difference between the two expressions.

The expression "阴魂不散" comes from the notion of a deceased person whose soul for whatever reason, remains in this world instead of moving on the the next one. Therefore, you can use the expression as a metaphor for someone with whom you no longer have any contact, but whose influence in your life is still present.

你们已经分手了！她怎么还是阴魂不散呢？

On the other hand, “百足之虫，死而不僵” refers to how a centipede's legs will keep moving for a while even after it dies, and can be used as a metaphor for instituions that don't die easily or that are difficult to erradicate. For example, you could use it in reference to how the influence of Confucianism on Chinese society survives, even after the Cultural Revolution, 破四旧, etc.

泡泡中文老师们，is there anything I've missed?