The issue is not that simple , i consider PostgreSQL more "enterprise" is just my point of view.
One thing i love about postgres is the Transactions + an extremely good engine , PostgreSQL is not made as easy as mysql you don't get fancy "phpmyadmin" but it does the work extremely well even in some corner cases.
You can find many pages like this one http://www.wikivs.com/wiki/MySQL_vs_PostgreSQL , anyway you can always test but from my experience , running FS clusters for three years PostgreSQL never let me down even when the RAID card failed , the internal triggers "locked down" the database , no rows were lost , the transaction engine rolled back current transactions and it saved my day :).
The lesson is , test first deploy later , PostegreSQL is for me , more feature reach and more stable but it requires more skills , mysql is more easy ofc less features and less stable ( this isn't true for all the cases).
Its up to you use something that you feel comfortable so when problems rise you can actually solve problems this means you understand the database and how it works.
P.S one point positive for mysql OPENSIPS tend to use mysql way more :)
There is also a more "oracle" version of PostgreSQL ,http://www.enterprisedb.com/
Regards
Antonio