UWP and Windows store are orthogonal (since 2016-17). UWP apps can be installed from the web or other stores and non-UWP Win32 apps can be in the Windows store.

UWP and Windows store are orthogonal (since 2016-17). UWP apps can be installed from the web or other stores and non-UWP Win32 apps can be in the Windows store. So "UWP versus Steam" doesn't really make sense.