Danger on Ruby is more mature, has more features, a solid plugin eco-system and covers more platforms. It supports Ruby 2.0 and above, and will likely work on any CI setup without additional work. For a lot of projects this is the right option.
The project is on version 5.x with about a hundred releases, that’s a lot of battle testing. I consider it in a great place and is unlikely to have breaking changes from this point onwards. The GitHub issues to number of downloads is really healthy too.
I have no intention of ever deprecating Danger Ruby.
Get started with Danger on Ruby.
You can create plugins today, and will be able to do things that the Ruby version could not - eventually.
Using Danger JS now will make it easier to move to Peril the server version of Danger, which gives you the ability to skip running Danger on CI, and instead have that feedback instantly.