it can be confusing, no question. Sensi does integrate with Echo. So does SmartThings. But SmartThings doesn’t integrate with Sensi.
If you only want to control the thermostat by echo, fine, you can get the Sensi.
But if, for example, you wanted SmartThings to automatically change the thermostat based on SmartThings knowing that you left the house, you could do that with a CT100 but not a Sensi.