What is the difference between backing up and restoring a Microsoft Office SharePoint 2007 site collection through stsadm tool and backing up through SharePoint Designer (as a .cmp file)?
stsadm backup operates on the site collection level and is useful for tasks such as migrating sharepoint services between servers. In the past it was used for migrating between versions of sharepoint. I'm not sure if the migration role will still be true. stsadm is only available to a Windows administrator.
The designer backup is available to any sharepoint site designer or site admin, and can be used for having a backup they can restore on their own, or used to relocate a site or subsite to another branch of the site collection. This is a more end user tool.
The two backup formats are not interchangable with each other.