* Redis is a lot simpler and more dynamic since it isn't intended to solve petabyte problems or used on large clusters of computers. An example, you can play with Redis online: http://try.redis-db.com/

* Redis is much easier to understand, since Redis is a key-value store with rich datatypes (such as lists, sets and hashes). Cassandra is a database paradigm shift build to solve Google scale problems.

* Redis perfomance is similar to memcached - and memcached's perfomance is pretty amazing.

I would only pick something like Cassandra if I had to store a lot of data on a large number of computers. If I wanted to do something fast and I know that my data won't grow to terabytes then Redis is a very good choice.

Redis has a rapidly growing list of data operations to model a wide variety of problems within a bound storage space (the VM feature expands that space). Cassandra does a specific task, unlimited high volume redundant writes, extremely well. I use Redis for session data, caching and queuing needs but for the huge volume data store Cassandra is the answer.

Redis is a swiss army knife, Cassandra is a Ka-Bar.