Both Ably Realtime and Amazon SNS are used to solve fairly different problems:
Pub/Sub - the primary difference
Ably comes with a variety of client SDKs that can be used to connect clients directly to Ably's Realtime platform. All devices, be they servers or clients, are able to receive messages from Ably channels.
Amazon SNS is mostly used for server to server operations, with email and SMS being the only two ways in which message can reach the end clients. SNS lets your AWS services, such as Amazon EC2, Amazon S3 and Amazon CloudWatch, publish messages to certain SNS topics. These can then be used to trigger other AWS services such as Lambda or SQS.
Although topics in SNS are analogous to channels in Ably, they are both used for different purposes as explained above.
Other realtime features