Postgres advantages:
- Ease of installation and administration
- Cost
- Portability
Oracle advantages (for web development):
- Outer joins
- Ability to add columns with constraints to existing tables
I've found both Oracle and Postgres communities to be very helpful solving problems, so to me that's a tie, along with stability.
Postgres is implementing outer joins for the next release; if you're not planning to go live for a few months, you might save yourself some trouble by developing with the most recent unstable postgres for that reason. They're "real close" to beta now, with production release presumably to follow soon.
