ORDBMS vs OODBMS

Hello folks

I'm an engineering student and I would be very interested to learn the pros and cons between
these two DataBases:

- Versant : commercial OODBMS (Object-Oriented)

- PostgreSQL : open source ORDBMS (Object-Relational)

(Other DB like Oracle are ERDBMS and not ORDBMS if I guess
right)

It seems to me the main disadvantage of ORDBMS is the still
presence of Impendance Mismatch, which is completely absent
in OODBMS.

Well, I would like to focus the meaning of the "Object" point
of view of PostgreSQL: does it offer just the arbitrariness
of data structure? What else?
Can a row be assimilated to an object ?

Please let me know any experiences which can show interesting aspects of
PostreSQL vs Versant (and/or other OODBMS in general) ?

The good thing in Versant is that it's 100% Object-Oriented
and works well with Java (through JVI interface). The sad thing is that it's
closed-source and it's very commercial.
The good thing in PostgreSQL is that it's open-source and
is available everywhere. I hope to find a lot of more good
points related to PostgreSQL.

Regards
Nico