Merits of a “Message Passing” system vs. an “Event Based” system

Question

My question is coming from an somewhat uneducated perspective.

What are the relative merits of a "message passing" system vs an "event based" system.

Why would one choose one over the other? What are their strengths and weaknesses?

I would like to know not just "in theory", but also "in practice".

EDIT:

Specific problem:

I want to build system of plug-able components which behave as small scale services (each performing some small task and or providing some piece of information).

The services can:

• be layered so that the output of one service can act as one of the inputs to another
• have containment hierarchies so that one service can be contained by another without a specific input-output relationship as mentioned in the previous sentence

The goal of the system is to translate a single flow of low level events in another system into higher level information and functionality, and in addition provide a channel back to the other system providing a single series of events.

I can provide more detail if this isn't enough.

After looking around some more. This and this probably describes my situation better.

This looks like it may be a good fit for my situation: http://akka.io/