You will have to use the phone to make LTE into something the Auto Echo can use to get to the net. So using your phone directly is easier. Especially on the Model 3 where you just plug it in, into the center console, giving you direct-view of the controls, and launch Sirius, or ask Siri or Google too. No issues with the phone not being available via Bluetooth for calls and msgs as there would be if you had the Echo using that BT connection. The phone also has a scrolling list of what's on the other XM channels, the Echo will not.
The big advantage of satellite has always been access to content when not in a major town, the network connection to your phone will fail at the exact same time as the network connection to your car, while satellite will typically fail in major urban settings where there is LOTS of cellular.
Not that I plan to renew Sirius on my other car, I am spending all my time in the Tesla. I just bring a USB stick full of music for the drive down Hwy 1.
-Randy