If your JMS provider supports JMSPriority (they are not obliged to), the jms-backed channel should work for you out of the box - it sets the JmsPriority using the MessageHeaders.PRIORITY in the SI message, when a message is sent to the queue/topic backing the channel.
AMQP does not have a concept of message priority.
You can simulate it by having n AMQP-backed channels and route to them (e.g. HIGH, MED, LOW). It's only a simulation because it doesn't prevent LOW or MED messages being processed before HIGH but, as long as the ratios are reasonable (small HIGH, med MED, and many LOW), the simulation is reasonable. You can further tweak things by altering the number of consumers on each channel (having more consumers on a HIGH priority channel etc).
Gary P. Russell
Spring Integration Team
SpringSource, a division of VMware