Name some differences between Express vs Recoverable messages

• Express messages are stored in RAM during routing and delivery. This means that if the machines goes down, the messages are lost. Their main advantage is that they provide better performance.
• Recoverable messages are stored on disk. This means that messages are not lost when the host machine goes down. Of course, this comes with a performance penalty.

Transactional messages have to be recoverable. Transactions incur some overhead, so you can expect less throughput when using this type of messages.