What's the difference between 圣人 and 神人 in ancient daoism?
Let's look at some references to the two to see the difference. Here's an early reference from Zhuangzi, Chapter 1, 内篇：


  至人无己，神人无功，圣人无名。


Or in translation:


  The perfect man is without self, the holy man is without achievement, the sage is without name.


To quote from Baidu:


  早期道教经典《太平经》就将神仙分为六等：一为神人...五为圣人，。并称：“神人主天...圣人主治百姓。”


To paraphrase:


  The early Daoist text Taiping Jing classifies various beings into 6 classes: First is the holy man...5th the sage. The holy man holds up the heavens...the sage holds up the common people.


It's a difference in scope and the initial state of being. The 神人 is a being who is born perfect. They don't need to achieve anything to reach the Dao. The 圣人 is someone who worked to achieve the Dao. They are a person who can help others.