Rails or clone on Windows Server 2008 – Most Performant

Mmh, I don't know how to say this. Some people might bash me over this.

Language (Groovy and Ruby)

As a language I reckon Ruby is more funky compared to Groovy. Groovy only exists to ease Java programmer as you don't need to learn too much new syntax. But overall I reckon is not as funky as Ruby. Groovy wouldn't be the JVM language that is worth to learn based on attender's vote in this year's JavaOne but instead Scala is the one to go. Besides that, the original creator of Groovy himself does not have faith in the language he created himself in the first place.

Community and Job openings

As for the community, Grails community is not as big as Rails, though since the acquirement by Spring more and more people are using it in serious application. Rails has more job openings in the market compared to Grails (that is if you want to invest in looking a new job).

The framework (Grails and Rails)

But, as a framework, if you really care about maintainability and need access to Java framework and legacy Java system, Grails is the way to go as it provides cleaner access to Java. Grails itself is built upon several popular Java framework (Spring & Hibernate). Rails itself IMHO is funky like Ruby itself, but it's funkyness costs you maintainability. Matz himself prefers Merb over Rails 2 because Rails create a DSL on top of Ruby which is really against the Ruby philosophy. And I reckon because Rails itself is opiniated, which in turn if you don't have the same opinion as the creator, it might not fit your needs.

Conclusion

So in your case, learn Grails as that is the company's consensus (you need to respect the consensus) and if you still want to secure your job. But, invest some time learning Rails and Ruby too if you want to open a chance getting a new job in the future.

You can make a career of front-end user interface development without know a ton about server code. You would do well though to have at least a rudimentary understanding of what happens on the server when you send it a request, where your data comes from, and what the life-cycle of a web page is. This assumes that you have the support of back-end developers. As you mentioned Ajax in your question that implies that you want your web sites to actually do something, which will require things to happen on the back-end (such as storage, manipulation of data, logging in a user, etc.).

As with all things, the more you know, the easier it will be to get what you want from the dedicated professionals. I would suggest that you learn about programming in general, not try an learn a language and framework. In particular, try to understand datatypes, server settings (like timeouts, post versus get, etc.), security and database interactions as they exist beyond JavaScript/ECMAScript. That way when a developer is explaining why they cannot do something you have requested or are offering alternatives, you are speaking the same language.