When configuring a user input task in a process model, what must the data from the form be saved into in order to pass it to another activity? is it rule input or node input?

If you want to pass the data from the interface into process model and then again to another interface. you have to follow below steps:

1) create rule inputs in interface.

2) store the value of those input into process variable and then pass those variable into another interface as an input.

I recommend the tutorials to get a better understanding of how this works.

docs.appian.com/.../Tutorials.html

The data is saved from Rule inputs to Activity Class variables (AC). Now when the data is in AC, it is accessible only within that node and the data will be destructed as soon as the node completes. If you want to carry this data forward to the next node, then you have to save it in Process Variables (PV). 

Saving data from RuleInputs/NodeInputs to AC is done on the "Form" tab.
Saving data from AC to PV is done on the "Data" tab.