It depends on how often you plan on changing the XSD file.
-If the file generates and compiles new *.cs files at runtime, use an
external script. This will help avoid attaching the *.csproj file in a
production environment which is a huge no-no.
-If you're generating a one-time *.cs file, do so at build-time, I'd
suggest keeping it in the pre-build script. Yeah, you'll be generating
and compiling it every time, but you'll avoid moving any source files
into a production environment. In case the XSD changes, your build
reflects them automatically instead of having someone know to generate
the *.cs file every time the XSD changes.

goodluck
-tom