Is it better to have many small Azure storage blob containers (each with some blobs) or one really large container with tons of blobs?

I don't think it really matters (from a scalability/parallelization perspective), because partitioning in Win Azure blobs storage is done at the blob level, not the container. Reasons to spread out across different containers have more to do with access control (e.g. SAS) or total storage size.

See here for more details: http://blogs.msdn.com/b/windowsazurestorage/archive/2010/05/10/windows-azure-storage-abstractions-and-their-scalability-targets.aspx

(Scroll down to "Partitions").

Quoting:

Blobs – Since the partition key is down to the blob name, we can load balance access to different blobs across as many servers in order to scale out access to them. This allows the containers to grow as large as you need them to (within the storage account space limit). The tradeoff is that we don’t provide the ability to do atomic transactions across multiple blobs.

Everyone has given you excellent answers around accessing blobs directly. However, if you need to list blobs in a container, you will likely see better performance with the many-container model. I just talked with a company who's been storing a massive number of blobs in a single container. They frequently list the objects in the container and then perform actions against a subset of those blobs. They're seeing a performance hit, as the time to retrieve a full listing has been growing.

This might not apply to your scenario, but it's something to consider...

Theoretically speaking, there should be no difference between lots of containers or fewer containers with more blobs. The extra containers can be nice as additional security boundaries (for public anonymous access or different SAS signatures for instance). Extra containers can also make housekeeping a bit easier when pruning (deleting a single container versus targeting each blob). I tend to use more containers for these reasons (not for performance).

Theoretically, the performance impact should not exist. The blob itself (full URL) is the partition key in Windows Azure (has been for a long time). That is the smallest thing that will be load-balanced from a partition server. So, you could (and often will) have two different blobs in same container being served out by different servers.

Jeremy indicates there is a performance difference between more and fewer containers. I have not dug into those benchmarks enough to explain why that might be the case, but I would suspect other factors (like size, duration of test, etc.) to explain any discrepancies.