I would probably go with postgres if your database is
not too large (and to me too large is probably something
like 5gb), didn't need replication or other high availability features of oracle, and could get along without the
permissioning, table partitioning, and other fancy
oraclisms (all of which are not really used by acs/openacs
anyway, unless you have added them).
That said, I know people at redhat are deploying postgres
as HA solutions (though not with replication as far as I know) and postgres seems to be gaining ground quickly.
I have been really beating on postgres to see what breaks
and it has held up really well. I have been writing up the
stuff I did specifically on forums tuning if you are interested.