When would I use a master page over a DWT? When dealing with an ASPX page
I'm not familiar with DWT...is that a Frontpage holdover? It might be a bit
like a Dreamweaver template in that it's an HTML file that has comments that
lock editing of certain areas via the IDE (in this case, Expression)
As for masterpages, consider them one specific templating framework that
happens to be included in the .net framework. They're OK, but limited, and
can cause issues if your application becomes complex with multiple controls
that need to communicate info.
Personally, I don't like how masterpages work in .net, so I just don't use
them and usually come up with my own templating systems. But they're
certainly a viable option.
-Darrel