Like you said there are several commercial solutions out there:
- Metalogix
- AvePoint
- Quest
Each has its own pros and cons.
My personal recommendation is to do an upgrade from SharePoint 2003 to 2007 on a test server with a copy of your content database. Then you can use STSADM to move the data or you could think about just moving the entire site collection by copying the content database.
We had to do something similar but our data had to come out of SharePoint 2003 and MS CMS. We looked at the commercial tools, but decided to build our own migration tool in the end.