The VERY simple explanation is:
With Ask Alexa you can define your invocation e.g. “Echo, ask my home to turn on the lights in the kitchen…” (“my home” is the invocation)
For Echosistant you create on skill per invocation e.g. “Echo, turn of the lights in the kitchen…” (“kitchen” is the invocation).
It is a lot more complicated and what you do will depend on your use case… that’s why I use both apps.
A recent discussion by the developers of each app in this thread will tell you more: