What is the difference between Esper and Drools?

  Hi,
I have a problem to consult you. You know Drools is a popular product of rules engine, and it also contains the technology  of Complex Event Processing, in China most companies  have been use Drools for a long time, and Drools has been in development . My problem is: compare with Drools, what advantage does Esper have? Or what differences and similarities  does the two products have? When my customer ask me the reason of choosing Esper but not Drools, how can I answer them? I need your help.
Very Thanks!

Best Regards!