Speed: how many concurrent connections are we talking about here? Given equal hardware, PG supports a greater number of concurrent connections without bogging down; and with the 8.0 release, speed is on-par with MySQL in most areas. Look for speed improvements in 8.1.
Security:
"By default, PostgreSQL is probably the most security-aware database available ..."
Database Hacker's Handbook
Lithcfield et. al.
Wiley
Wiley