The main page of ART is quite identical to RawTherapee, the differences are in the modules (functions) and - under the hood - in the processing pipeline. I'll focus here on the things that a photographer might be interested in.
Here's a comparison of the modules offered by ART and by RawTherapee.
ART left, RawTherapee right
ART left, RT right
ART left, RT right
ART left, RT right
ART left, RT right
The Metadata sections are roughly the same in the two programs.
As you can see, some modules of RT were stripped, like the rather complex (overly complex?) Wavelets, Retinex and CIECAM. Other modules were moved to another location, simplified, like Exposure, or added, like Log Tone Mapping and Perspective.
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2/2020