What are the big differences between IntelliJ + Ruby Plugin vs RubyMine?
The Ruby plugin and RubyMine are build from the same codebase, that's right.
Java/Ruby integration for JRuby code is only available in Ruby plugin for IntelliJ IDEA. But the plugin doesn't have RubyMotion support.
Also check this answer: If I have IDEA, do I need rubymine for ruby dev or will it do both?