So what is the functional difference between a stack decoupler and a stack separator? Why might I use one over the other? In what cases would I want to choose the TR-18D Stack Separator over the lighter TR-18A Stack Decoupler?
The Stack Separator decouples from both the child and the parent node whilst the Stack Decoupler remains attached to the child.
Why might I use one over the other?
Situations where you are separating satellites, rovers, probes or other payloads and don't want the Decoupler still attached.
Also, the decoupler have an explosive charge attached to it that might change the orbit of your ship if the gravity of the planet/moon is too weak and/or your ship is too light. I'm not sure if that's the case of the separator. If it doesn't change the orbit, that's another advantage.