Ask HN: How to choose the right in-memory NoSQL database?

I know that is an immature question. So I am going to throw in a few details: We are looking for an open sourced NoSQL database which can handle at least 5000-6000TPS with approximately equal number of reads and writes. We plan on using this for real time analytics (aggregate or map-reduce). In-memory would be nice for the obvious reason. We are currently using Redis and MongoDB. I love Redis, but I can't seem to hold on to it when people at my company mention the word "cluster". And about MongoDB, well it doesn't seem to be good enough for our read/write requirement. Choosing the right option seems to extremely challenging. I am not asking for you guys to point me at a database, but maybe provide a few options and advice for us to evaluate. Thanks!