What makes InfraRuby better than Crystal (or vis-versa)?

InfraRuby code runs in Ruby interpreters.

Cool. I assume that makes interop with Ruby easier?

Yes! Ruby language features supported by InfraRuby have the same behavior as Ruby interpreters.

InfraRuby projects are created with rake tasks to run your code with either the InfraRuby runtime or Ruby interpreters. You could implement a file format or protocol in InfraRuby, and use that code in a Ruby project too. You could use Ruby interpreters for development and the InfraRuby runtime for production.

And if you decide that InfraRuby is not for you then you can take your code with you!