Ecart is a tool for working with shopping carts and checkout functionality.
Powerstore is a completed site generated with most of the tools in the WebAssist suite.
The difference between powerstore and eCart is the difference between a house and a bulldozer.
If you are looking for a place to live, the house is the way to go. If you are looking to make a living building houses or really want to customize your house a lot, you probably would need a bulldozer.