Amazon Web Services: python or ruby?
From my understanding, there's no official SDK for python, is boto an adequate replacement?
Absolutely, boto could have been considered the de facto Python SDK for AWS for quite some time already, and recently its author, Mitch Garnaat, has fortunately joined the AWS team as well, see Big News Regarding Python, boto, and AWS:
Building on this model, Mitch Garnaat has also joined the team. Mitch has been a member of the AWS community for over 6 years and has made over 2,000 posts to the AWS Developer Forums. He is also the author of boto, the most popular third-party library for accessing AWS, and of the Python and AWS Cookbook.
Appendix: hazardous assessment
Your question regarding Which (AWS-related) community is bigger/better? can indeed only be answered subjectively, and I definitely agree with Marcin's comment to choose the language/development environment you are most comfortable with instead.
However, my utterly personal impression is, that Python might have an edge in the developer sphere, i.e. it seems to be more often the choice for facilitating AWS as a component in development scenarios (but then, I'm biased, insofar I prefer Python here as well and filter information accordingly).
On the other hand, Ruby seems to currently have an edge within the operations sphere, insofar the two (most?!) popular and widely used infrastructure automation tools, Puppet and Chef, are both Ruby based (but then, I'm biased, insofar I prefer these tools right now myself and filter information accordingly). Likewise both are explicitly addressed by AWS in their automation guides as well, see e.g. AWS CloudFormation Articles and Tutorials.
So choose wisely ;)
Scala can be third contender after Python & Ruby.
Many scripts will be best/easy to run using Scala, Python or Ruby instead Java/.net.