IBPhoenix is a nice little database, but it feels rough around the edges. If you look at the volume of messages on the PostgreSQL, IBphoenix, and MySQL mailing lists, you'll find that Postgresql by far has the most active development list.
More developers is a good thing.
I think IBPhoenix has a few features Postgresql lacks (no need to vacuum in IB, nor analyze I believe, which are a bit of a bother to the Postgresql beginner). Mayve clustering is there too for IBPhoenix?
Stored procs are supposedly a pain to develop in IBPhoenix, whereas in Postgresql you have a choice of a dozen or so languages, many of which are "trusted" I.e. sandboxed to prevent joe user from scrambling the database files while other languages are available that let you get out of the sandbox and do whatever you want (send email, delete files, etc...) as the postgresql master user.
If you are looking at Postgresql,
make sure it's either the latest stable (7.3.4) or the latest beta to get a fair idea of how it runs now, not a year ago.