I was in the process of evaluating nosql databases and finalised on two. RavenDB and MongoDb. Which one is better and why? Which will help in writing more performance oriented queries.
Raven's install base is smaller. When you consider the two, consider your growth path. Look carefully at how MongoDb and Raven differ on shard keys. Look carefully at support and community around the product - these will kick in once you have production critical issues. See if the indexing strategy and map-reduce constructs satisfy your use cases. Look carefully at the storage and latency of your largest collections and indexes. Decide whether the transaction model fits your needs.
If your application is simple enough, your abstraction over the DB layer should enable you to switch should you change your mind. But for a complex and robust application, I'd recommend a proof of concept before a full-prod commit.