I would argue assuming its stock trading, your concern should be transactions safety so you need a database with ACID support. Beyond that, which you choose is

I would argue assuming its stock trading, your concern should be transactions safety so you need a database with ACID support. Beyond that, which you choose is based on your experience. Speed is really not a concern anymore because you can use memcached or other distributed cached system when you really need that much scalability and fast access to raw data.