Why not ask yourself the question "Why Cassandra?" or "Why Riak?" or "why you are specifically looking at these two alternatives" for the task at hand. This way

Why not ask yourself the question "Why Cassandra?" or "Why Riak?" or "why you are specifically looking at these two alternatives" for the task at hand. This way you know what each of these could do for your situation. Once you have established that, it will be easier to look at other factors.