In my experience, go with RoR unless you're already in a deeply-committed Java shop. Rails has much more momentum & community support (even if Node is slowly chipping away at it.)

With Grails by comparison you're more likely to find plugins that haven't been updated in years and be constantly fighting dependency conflicts. At least that was my experience.

If you go the Grails route it's also inevitable that you'll need to learn all of the various "layers" underneath Grails nice veneer. E.g. Grails wraps a lot of existing Java technologies which are fairly complex on their own like JEE, JPA, etc. Rails seems to be "shallower" and easier to grok by comparison.

As an aside, I personally love the Groovy language and syntax too, and how easy it is to interop with any java libraries.

Don't get me wrong, if I needed to write a Java-based, user-facing web app, Grails is the first option I would look at. But short of the Java requirement I would choose RoR or even Sails.js (not as mature but rapidly improving it seems.)