I've been playing around with a trial version of Capture One lately. I had used it a while ago for my older Sony A200 RAW files. It was free to use with Sony cameras, but since I hardly ever shot past ISO 400 & the in camera jpeg engine did a pretty good job, I hardly ever post processed the A200's RAW files. Even then, it was a limited version of what the full version could do.
I use RawTherapee almost all the time when I process high ISO images. After being able to extract better high ISO results out of RawTherapee than what I could get out of Lightroom, I canceled my Lightroom subscription. I been playing around with RawTherapee even more since then. I recently decided to give Capture One another test drive just to give it the benefit of the doubt. I downloaded a trial version, installed it, & have played with it for the past 25 days. Well, all I can say is that it's pretty good. It produces a different kind of luminance grain at high ISO, it handles color better, & handles high ISO false coloring a bit better.
I don't know what kind of demosaicing algorithm is being used in Capture One, but it can render high ISO shots with a lot of detail & very fine grain. RawTherapee also renders high ISO shots with a lot of detail & very fine grain. They're about equal in that department. It all depends on the unsharp mask/sharpening settings that you use in each program, if you use the amaze demosaicing algorithm in RawTherapee, the green equilibration settings in RawTherapee, & the luminance/detail/single pixel sliders in Capture One. You will get very pleasing high ISO results from both programs. I can't really say that one is better than the other in that respect.
Color goes to Capture One. The color seems to be more accurate, has more saturation, & is more vibrant. That's where it beats RawTherapee. I can't seem to get those vibrant Capture One colors in RawTherapee. The vibrance slider in RawTherapee can kind of replicate similar colors, but then other colors might shift tint a bit. I still need to work on RawTherapee's colors. I think it's a combination of the chrominance noise reduction settings, the saturation values, vibrance settings, & L*a*b Adjusments in RawTherapee.
Capture One handles high ISO false coloring a bit better. RawTherapee does pretty well, but Capture One is slightly ahead. I'm talking about false coloring that you see randomly throughout the image at high ISO, not chrominance color noise. They both handle that well.
The control layouts are similar to how RawTherapee has them laid out. If you can use RawTherapee, you'll have no issues using Capture One. The way the control layouts are organized in Capture One makes it a bit easier to use, though.
I guess when you look at it, RawTherapee is pretty great for being free. It offers a ton of controls that can be overwhelming at times. If I were to purchase a raw editing program, Capture One would probably be my first pick & Lightroom my second pick. Would I pay for it? If I were shooting professionally & making a living off of it, yes. Since I don't do that, I won't be buying it anytime soon. It's a bit pricey for me at $299 USD, but it does offer a lot of control options for the price. You can also purchase a film simulation pack for about $50 USD. You can use HaldCLUT film simulations in RawTherapee for free, though. However, Capture One will still have a better color representation in the end.
I want to try out DxO next to see how that one does.