There are some similarities between the two; they both support JMS 1.1 and run inside JBoss 4.x.
However ActiveMQ does offer some specific differences and advantages (at least from our perspective)
There is a FuseSource tuning guide
If you're not convinced by performance reports then please do try running performance tests yourselfs. You might wanna check out our overview of Performance or try using out the ActiveMQ Performance Module Users Manual