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)
If you're not convinced by performance reports then please do try running performance tests yourself. You might wanna check out our overview of Performance or try using out the ActiveMQ Performance Module Users Manual
The Commercial Providers on the Support page may also be able to help diagnose performance issues, suggest changes, etc...