Are you referring to the Sophos Web Appliance, also known as the SWA?
No the SWA is not just a pared down version of the UTM, it is a different codebase. The SWA is a product that Sophos built itself. The UTM was originally built by Astaro, which was purchased by Sophos several years ago. These days the same development team does the web on both products.
One of the main purposes in 9.2 for the web team was to bring the UTM's policy configuration UI closer to that of the SWA, as well as bring the feature set closer.
The web feature set of UTM 9.2 and SWA 3.9.1 are fairly similar, however the SWA just had a release that brought out a few new features. The next UTM release will bring the features set closer still.
Not sure if this is a question for here, but can you use Protection Suite licences to licence within the UTM? e.g. complete comes with the Secure Web Gateway, Secure email Gateway, etc. - can these be used to activate the features on the UTM?
If not, is it planned in a future version to bring the different licence schemes together?