Angular is becoming very popular, and I think as a front end dev, you'll pick it up quickly.<p>As a full stack dev with experience with both Angular and Rails, and recently been interviewing, I think Angular will serve you better over the next few years.<p>There is currently a focus on pushing the envelope with front-end UX, single page apps, etc. etc.
I think a lot of the interesting work will be in that area. I suspect that over the next few years, the majority of back end work will continue to be pushed toward being APIs to serve the front-end javascript.<p>At the same time, having a server-side language in you toolbox is a good thing. But why rails? You already know javascript, to it seems to me the obvious path would be node.js. It is becoming extremely popular, and will allow you to leverage your existing knowledge of javascript.<p>Beyond everything else I've just said, I've recently started programming with node-webkit (I'm making an IDE) which further expands your horizons to installed apps to desktop environments.<p>Though there are a lot of haters out there, there is a huge future in Javascript, Rails is popular in certain circles, and it's a great language, but I wonder how popular it will be in another 5 years.