Both kinds of Virtual Destinations offer similar capabilities, namely that the producer sees a single Destination to send to and the consumer sees a different Destination to consume from and there can be a many to many map of producer Destination to many different consumer destinations (as well as a mix of Topics and Queues.
The main difference between a Virtual Topic and a Composite Desetination is that with a Composite Destination the list of consumer destinations is static and hard wired. Whereas with a Virtual Topic at run time a new consumer or queue can be created dynamically and addded to the subscription without having to reconfigure the broker.
e.g. for a virtual topic, VirtualTopic.Orders at run time a new consumer can be created on the non-existent queue Consumer.FOO.VirtualTopic.Orders (where FOO is the unique name of the consumer) and the queue will be created which is also automatically wired up to the virtual topic as a consumer queue.