All very interesting.
Here’s a preliminary comparison summary, coming from a beginner’s amateur perspective rather than professional.
Both are a gift. Not only free but both are good softwares too.
RawTherapee is easier for beginners to dive into, for various reasons I won’t list just yet. But RawTherapee does (also) have strong stuff under the hood too, once you are ready to go there. It is easier for newbies to get started with.
Darktable is far harder to learn, perhaps because there are so many optional choices. The right side menu options often scroll off the screen for me (the yellow mask toggle button disappears for me when manipulating sliders at the top of the exposure menu. That’s probably because I’m old and have bad eyesight, and had to increase font size. That took a week to figure out. The vertical right-side menu options slider only appears on mouse over, so it’s hard to use, even when you know it’s there. On balance I am in love with Darktable.
I have 10 year old digital images that never looked so good.
The biggest difference, by far, between the two worlds is masking, which comes with the mix inside Darktable but not (not yet anyway) in RawTherapee. But that doesn’t mean you can’t use masking in RawTherapee. My workflow that way is to create various layers and masks inside Gimp and then to apply them to disparately edited versions of the same raw in RawTherapee. I like being able to create black and white masks in gimp and perhaps manually edit them with various brushes, shrink, expand blur etc. And then to reuse that same mask multiple times, for exposure, contrast color correction sharpening blurring etc. I can’t do that with the Mint Linux (set ppa update) apt-get install darktable I work with.
Global sharpening for long lens telephoto wildlife photography is a disaster because you still want a nicely blurred background. Masks are not optional. The only question is who how where and what way to use them.
Do I like one direction better than the other? Too soon to say. I use them both now. Thank you developers.