A customer is evaluating the Platform Events solution and would like help in comparing/contrasting it...

In both Platform Events and Outbound Messaging, the event messages are retried by and delivered in sequence, and only once. Salesforce ensures there is no duplicate message delivery.

Both Platform Events and Outbound Messaging are highly scalable. However, unlike Outbound Messaging, only Platform Events have Event Delivery and Event Publishing limits to be considered.

Message sequence is possible in Outbound Messaging, but not guaranteed with Platform Events. Both offer very high reliability. Fault handling and recovery are fully handled by Salesforce.