Just based on looking at the sites, Rawtherapee appears to focus on RAW "developing", while Darktable appears to also do photo management and Lightroom-style non-destructive editing.
RawTherapee does photo management and is non-destructive as well.
Darktable supports masks and parametric masks for almost all operations. RawTherapee on the other hand has a wider range of tools and support for profiles and raw formats.
RawPedia's "Features" page (http://rawpedia.rawtherapee.com/Features) doesn't mention photo management, and I didn't find anything by searching for "catalog" (Lightroom's name for a collection of managed photos), etc. It'd be great if the site had more info about that aspect of the app.
The equivalent functionality is the File Browser tab when you open RawTherape. You can filter images, assign ranks/color labels, create queues, do batch edits, assign a dark frame, etc etc.
It doesn't have "collections" or "catalogs" per se, if that's what you're specifically looking for.
