The Information Schema vs the PG Catalog

I'm writing PL/pgSQL routines that generate triggers, functions, and rules
based on design characteristics of tables, columns, and other database
objects. These routines need to be able to look up the definitions of these
objects. I see that there are two places available to look up this info:
the Information Schema and in the PG Catalog.

Which source is preferable? Or if that answer isn't absolute, what are the
reasons or conditions for preferring one over the other?

Also, a specific question: Does the Information Schema offer any way to
list the sequences that exist and their attributes? I can't seem to find
any.

~ TIA

~ Ken