In your case, if you are just planning to de a web API with the majority of the workload being I/O (such as hitting other APIs), then node.js (outperform Python and Ruby out of the box in this case IMO). Otherwise, Python is a more versatile choice. All 3 are relatively easy to start, but given you like Rails, which is big on convention over configuration, the closest you'll get, that has a large community is Django, and a Node.JS counterpart would be sails.js. The biggest difference is in the languages itself, all 3 have some distinct differences in design.

But all 3 suffer from the same issues that dynamic languages have, and weren't designed for enterprise. Node.js is also being added to existing enterprise applications, but normally for anything web facing.

EDIT: Add opinion relating back to main question.