I would think both "startup script" and "software installation" can be used to install software.
Via the startup script, I had to find some checks to avoid installing software over and over again, like looking for the uninstall key in the registry.
Microsoft also has it's ways to determine if software has been installed. There are things (advantages?) like the possibility to redeploy a package, or the possibility for the user to repair the software.
http://support.microsoft.com/kb/816102
http://technet.microsoft.com/en-us/library/bb742421.aspx
A few years ago, we deployed Office 2003 with some MUI via GPO, Software Installation under Computer Configuration. I think the only issue we had was lack of space on some computers (6 GB Hard Drives...).
About eight moths ago, we deployed new computers with Office 2003 in the image I created.