They are more complimentary and in fact storm has a spout implementation for kafka I would add to this that a lot of Storm users (myself included) use Storm to

They are more complimentary and in fact storm has a spout implementation for kafka I would add to this that a lot of Storm users (myself included) use Storm together with Kafka - that is, Kafka is used to get data into (and possibly out of) Storm while Storm does the actual processing. Kafka is more along the lines of Kestrel and RabbitMQ.