I'd say pick the one that's compatible (or has great libraries) for what you are working in (eg. .NET, Rails, Django etc.). Especially in the beginning of a sta

I'd say pick the one that's compatible (or has great libraries) for what you are working in (eg. .NET, Rails, Django etc.). Especially in the beginning of a startup project, it's important to maintain efficiency. From what I've seen, most start out with a RDBMS and switch to hybrid RDBMS/OODB later on. I've also seen some switch entirely to OODB from RDBMS and back into a hybrid. There's also a case for schemaless document oriented databases like MongoDB. Honestly, I haven't seen 'serious' uses of document oriented databases other than for distributed caching. In general, 'big' data needs structure. One of days it might even become common to see most browsers support indexeddb (sql like) and local storage (kvp like). So back to my point; efficiency, compatibility and familiarity is key for rapid application development, which is a must have for start-ups.