Having worked with Ruby on Rails in the past and with Play2 and Scala currently, I think I can answer that.
For your regular web app, both frameworks are mostly the same, both are boring - Rails is more productive for very common tasks, but Play2 is safer because of the underlying language (well, if you pick Scala, otherwise Java just stays in your way). Things get more interesting in Play2 when scalability or latency starts to matter.
That's when you discover that Play2's architecture is entirely asynchronous and that Play2 supports asynchronous responses and web sockets natively. It does so by means of futures , actors  and iteratees  (also being migrated to reactive streams ).
Of course, the easy route with Play2 would be with Java, however I recommend that you pick up Scala. By doing so you're going to get exposed to concepts from functional programming in a language sitting on a platform that's very much practical for real world use. There's also this book on functional programming in Scala that's amongst the best books written on the subject . Functional programming changes the way you think and it's all about sanity, functional code being easier to test, being easier to parallelize and being much less prone to accidental bugs.
But while you're at it also consider learning Haskell, probably the best functional programming language available, because even if you're not using it, it's currently the lingua franca for FP concepts, so when reading papers and blog articles on interesting design patterns related to FP, most of them are described in Haskell. It also spoils you with its incredible type system, so a language like Scala will become the minimum that you'll tolerate, working in Java, C#, Python or Go becoming unbearable ;-) A really good beginners course is this one from edX . And then go learn Clojure, because it's a really practical LISP that is also oriented towards FP, except that FP in a LISP is really different from FP in static languages like Haskell or Scala.
And you know, the best thing about this path is that you're not going to learn just a framework, or yet another language, but you're going to learn about functional programming, which is a concept that transcends programming languages with its related mentality and design patterns and is useful no matter what you're doing and in what language.
1) Functional Programming: What? Why? When? by Robert C. Martin @ https://vimeo.com/97514630
2) Erik Meijer Introduction to FP @ https://www.edx.org/course/introduction-functional-programmi...
3) Do you want to learn F# and Functional Programming? Well, you better start coding! @ https://github.com/jorgef/fsharpworkshop
I want to note that LYAH may not work for you if you have my kind of learning style. I need to solve concrete, realistic problems to creatively use the knowledge imparted on me. I can't just read stuff and understand. Real World Haskell , the course by Erik Meijer  and the courses referenced here  worked much better for me.
For newcomers I'd suggest this introductory course by Erik Meijer: https://www.edx.org/course/introduction-functional-programmi... . It's about FP, not strictly Haskell, but it really helped me starting with it and really stimulated my curiosity (and he really can explain hard concepts in a simple way imo). I started reading the book after I completed the course and helped giving an answer to a lot of questions I had during the course.
What's the percentage of edX courses that are actually CC licensed?
As a quick and unscientific experiment, I checked out the 12 courses listed on the edX.org homepage and none of them were CC licensed.
With Google I found a few CC licensed edX courses [0:2], but looks like they're the rare minority.