I had a partner ask the question of the differences between the 2 clients and I didn't know, so you can thank Henry for finding it.

From the link, look to the left navigation for these other items -

Getting Started
Overview
What's new for corporate accounts in Messenger for Mac 7
System requirements for Messenger for Mac 7
Upgrading and interoperability
Messenger for Mac 7 compared to Office Communicator 2007
Architecture and Planning
Messenger for Mac 7 topology
Security best practices
Installing and Configuring Messenger 7 on a Corporate Network
Installing Messenger for Mac 7
Connecting to Office Communications Server
Uninstalling Messenger for Mac 7
Post-Installation
Setting audio/video preferences
Deploying Messenger for Mac 7 preferences
Using Messenger for Mac 7 with Office 2008 for Mac
Technical Reference
Messenger for Mac 7 preferences