Question

They both let you stick some mako-rendered content into another one. Both let you pass in arguments.

Is there a performance difference between the two? A philosophical one?

Answer 1

Def is for defining functions in the template, while include executes another file.

The mako documentation summarizes thus:

Calling a <%def> from another template is something like using an
<%include> – except you are calling a specific function within the
template, not the whole template.

(from http://docs.makotemplates.org/en/latest/defs.html)

Of course, <%include> involves another file, you can call a <%def> from the same file it is defined in.