Hello hoorayhooray,

If I were you, I'd go with the MCAS since it is based on Office 2007. Office 2003 is becoming more and more outdated as we speak (especially since Office 2010 is releasing next month!). However, if you are aiming to impress a specific employer and you know they are using Office 2003, MOS might the way to go.

Here is a thread where people discuss both certifications:
http://social.microsoft.com/Forums/e...b-599bbfbca0ac

Cheers,
Cassandra
Microsoft Office Outreach