Which NoSQL DB is best fitted for OLTP financial systems?

We're designing an OLTP financial system. it should be able to support 10.000 transactions per second and have reporting features.

So we have come to the idea of using:

• a NoSQL DB as our main storage
• a MySQL DB (Percona server actually) making some ETLs from the NoSQL DB for store reporting data

We're considering MongoDB and Riak for the NoSQL job. we have read that Riak scales more smoothly than MongoDB. And we would like to listen your opinion.

• Which NoSQL DB would you use for a OLTP financial system?
• How has been your experience scaling MongoDB/Riak?

There is no conceivable circumstance where I would use a NOSQl database for anything to do with finance. You simply don't have the data integrity needed or the internal controls. Dow Jones uses SQL Server to do its transactions and if they can properly design a high performance, high transaction Relational datbase so can you. You will have to invest in some people who know what they are doing though.