According to each provider, those products are described in the following way:
WSO2 Enterprise Service Bus is a lightweight, high performance, and comprehensive ESB. 100% open source, the WSO2 ESB effectively addresses integration standards and supports all integration patterns, enabling interoperability among various heterogeneous systems and business applications.
Mule ESB is a lightweight Java-based enterprise service bus (ESB) and integration platform that allows developers to connect applications together quickly and easily, enabling them to exchange data.
I personally did not find those descriptions much different or useful to distinguish each product. If anyone used both and would like to share her/his personal experience, please comment and help us choose the best system for our applications