Pycassa is an older python driver that is thrift based while python-driver is a newer CQL3 driver based on cassandra's binary protocol. Thrift isn't going away but it has become a legacy API in cassandra so my advice going forward is to use the newer python driver.
I wrote a blog that you might find helpful, which uses the Twissandra example application with the DataStax python-driver to provide an overview of CRUD and using prepared statements etc.
As for cql I haven't had any experience with this one, but the homepage of the project says it all:
This driver has been deprecated. Please use python-driver https://github.com/datastax/python-driver instead