RabbitMQ can do much of what Workq can do from a purely messaging standpoint since input and output looks about the same.

Workq is built on the higher level concept of a job so the feature set is refined around what a job is. In Workq, a job must successfully complete or fail, and optionally a result passed back to the client. A job can be retried when it has timed out or even when it has explicitly failed outright (maybe there was an temporary error with your API provider..etc). You can say: retry the job if it has timed out up to 5x BUT let it explicitly fail only once. These small refinements help streamlined the concept of processing a job fully, not just a blob of message.

Also there are some other key things such as job scheduling based on time which don't exist in RabbitMQ, but are usually offered in libraries such as DelayedJob..etc.