Ask HN: Groovy/Grails or Scala/Play?
if I had to pick one to go with and learn, what would be my best option?
Based on experience, you can deploy 'something' out there and build that 'something' really fast with Grails. GORM instantly takes care of persisting to DB with minimal effort, there's not much to configure. If you're planning to just create an API Server, the latest Grails 2.3.x releases have specifically made that easier too.
Can't say anything for Play since I haven't used that, but if you face growth problems with your Grails project (scaling), just piece off the heavy parts and create separate services written in Scala. First hand experience, Finagle is a good choice for this. You can easily be productive and quickly create services with this RPC system.
Conclusion: In my opinion, your best option for creating an MVP/Prototype is Grails.
The problem is I really don't know the underlying technology and I'm going to hit several walls really soon :)
I chose it because I know a friend who works with it and is really happy and evangelizes a bit :) . I never considered Scala/Play. My other option was MVC4, but .NET is pretty expensive for an MVP (not many options for free or cheap hosting, etc.) and I wanted to explore other options.
Here are job trends for these languages: http://www.indeed.com/jobtrends?q=Groovy%2C+Grails%2C+Scala&...
I've heard good things about it too :) .