For a some simple cases it seems to me that there would be no important difference between
- putting some code in a file
preamble.texand having\input{preamble.tex}in the main .tex file - putting it in a file
preamble.sty(with appropriate modifications such as a\ProvidesPackage)
What are the pros and cons of each?
\makeatletterand\makeatotherto surround instructions that contain the character@in the method based on\input preambleis one such difference that hopefully nobody will consider to be major. Far more important would have to be the decisions as to real stuff that should be in the file preamble.tex/preamble.sty. – Mico Oct 16 '13 at 18:08