Matt,
The Microsoft Assessment and Planning Solution Accelerator (MAP) is meant to be used to ease pains around desktop and server migration as well as virtualization. We encourage users to use MAP for one-time migration and NOT as an on-going management tool.
That being said, let me explain some of the key high-level feature differences here.
MAP
* Agent-less remote inventory
* Designed for one-time migration and deployment of OS and virtualization
* Out-of-the-box pre-defined Excel and Word reports for Windows Vista Readiness, Windows server 2008 Readiness, Virtualization Candidacy, as well as Office 2007 hardware assessments
SMS
* Agent-based inventory
* Designed for on-going management and operations
* Reporting is possible but will require customization efforts
Hope this is helpful!
Baldwin Ng
Sr. Product Manager, Microsoft Assessment and Planning 3.0
http://www.microsoft.com/MAP
BNg