The difference between SQS, RabbitMQ, and Memphis is in their cost, how they work, and how to access them. 

SQS is short for Simple Queueing Service.
It is from Amazon.
It is available with other services in the AWS cloud. It is only a message queue.
That is, it doesn’t have complex activities like schema validation and routing like you will see in a standard message broker.
SQS is not free. It is not open-source either.

RabbitMQ and Memphis are complete message brokers.
They are open-source. They are free.
While RabbitMQ uses exchanges and queues for routing, Memphis uses stations for the same purpose.
In contrast to SQS; RabbitMQ and Memphis have important message broker features like schema validation, message routing, and monitoring.