Rebol, is mostly "dead", and I don't like to so easily throw the term around. But, originally it started out as a proprietary commercial language, then when it couldn't make enough money, went open source. It has some good concepts, and passionate followers, but didn't go mainstream. Even though it went open source, in the flood of so many languages that exists now, continual development stalled. However, it did produce several offshoot descendant languages.
Something to point out, is there is a lot of old tutorials and information on Rebol that becomes more useful as references, if a person learns any of the descendant languages.
Red (https://www.red-lang.org), is a very good open source offshoot. If a person is choosing between them, I think most would recommend it. But, the problem with Red is its development can be described as sluggish. The lead developers appear overly preoccupied with other projects, as oppose to getting Red into a more useful state and delivering on the stated goals and expected features.
For example, one of its main benefits is easy cross-compiling to other OSes. However, it's stuck in 32-bit, and needs to have a 64-bit version to keep pace with the requirements of macOS, iOS, and Android.
If Red's development was to keep on pace, which is important for multi-OS usability, I do think it would be worth knowing. Quite powerful and easy to learn language, for potentially a wide variety of purposes, with a light footprint.
In reality? None. Both give me lots of trouble. Red doesn't offer a good external program launch, which when things fail, I always drop down to sh. Rebol interestingly enough offers a wee bit more stability. but its age is clearly showing. r3 being open source has a bunch of versions and forks, none of them usable. there's also ren/c which is supposed to be usable, however I cannot build it on a pi4, there is also another attempt which is called arturo, written in nim, this one works but development is only done by a single person.
Something to point out, is there is a lot of old tutorials and information on Rebol that becomes more useful as references, if a person learns any of the descendant languages.
Red (https://www.red-lang.org), is a very good open source offshoot. If a person is choosing between them, I think most would recommend it. But, the problem with Red is its development can be described as sluggish. The lead developers appear overly preoccupied with other projects, as oppose to getting Red into a more useful state and delivering on the stated goals and expected features.
For example, one of its main benefits is easy cross-compiling to other OSes. However, it's stuck in 32-bit, and needs to have a 64-bit version to keep pace with the requirements of macOS, iOS, and Android.
If Red's development was to keep on pace, which is important for multi-OS usability, I do think it would be worth knowing. Quite powerful and easy to learn language, for potentially a wide variety of purposes, with a light footprint.