There are more capabilities for controlling the Ecobee via SmartThings than with native Echo. There is a very robust ST development community and that platform is much more open and capable than just Echo by itself. You get the best of both worlds by using Echo for it’s strength (voice commands) and SmartThings for home automation control.
If you get a ST hub, I highly recommend @yvesracine DTH for maximum flexibility with the Ecobee.