Hello David
EWC runs on modern IOS-XE platforms, including the newer Cisco Catalyst 9100 Access Points. This is the same software platform as the 9800 series controllers. So it’s a scaled down version of the same software as the independent controller running on the switch, but it runs on the specific series of access points.
Cisco Mobility Express is based on the older AirOS platform and lacks many of the advanced capabilities of EWC.
I hope this has been helpful!
Laz