We're setting out to build an online platform (API, Servers, Data, Wahoo!). For context, imagine that we need to build something like twitter, but with the comments (tweets) organized around a live event. Information about the live event itself must be delivered to clients as fast and consistently as possible, while comments about the event can probably wait a bit longer to be delivered. We'll be read-heavy after the live event finishes.
Scalability is very important. We want to start out renting VPS slices, and scale from there. I'm a big fan of the cloud, and would like to remain there as long as possible. We'll probably be using ruby.
I'm convinced that I want to try a document store instead of an RDBMS. I like the idea of schema-less storage and the promises of easier scalability by focusing on key-value.
The problem is I don't know which technology is the most appropriate for our platform. I've looked at Couch, Mongo, Tokyo Cabinet, Cassandra, and an RDBMS with blobbed documents. Any help picking the right tool for this particular job?