What are the detailed differences between packed designs (.rsd) and start design template (.rsdt)

We use Windchill with the Windchill WorkGroup Manager, so all of our designs are .rsd packed design files.

We started with Schematics 2, using .rsd files for the templates.  This was before .rsdt files showed up.

What does moving to .rsdt files for our templates do for us?  IE: what features do they have that .rsd files do not or other benefits?