postgres vs. oracle for very large tables

I've written some extensions to postgres to implement
chemical structure searching.  I get inquiries about
the performance of postgres vs. oracle.  This is a huge
topic, with lots of opinions and lots of facts.  But,
today I got some feedback stating the opinion that:
"Postgres performance diminishes with large tables
  (we’ll be going to upwards of hundreds of millions of rows)."

Is this pure speculation, opinion, known fact?
Does anyone know of measured performance of postgres
vs. oracle, specifically with very large tables?

TJ O'Donnell
www.gnova.com