Postgresql and Interbase SQL portability

Does anyone know the differences between the SQL in PostgreSQL and
Interbase?  What are the problems in porting between the two.


Also, I am looking for samples on using the geometry datatypes.
Specifically I am interested in putting in points and then quering for
points that lay inside a given circle.

I am interested in postgresql because of these functions and I wish to write
my entire application in that DB.  But I am also interested in Interbase
since it has a replication story and may need something of the sort for
performance issues.

Thanks!